That doesn't sound encouraging. It sort of implies that nothing grows there, and if so, there is a reason for that.that dead gray brown dirt of the high dessert.
If that isn't correct and there is grass, how high does it grow? How high do your flowers grow?
What altitude is this? Many plants have very hard limits on where they will grow, based on altitude.
Do a dry run and do 2 things: get some soil samples to send to the AG extension for testing to find out what the soil lacks and bring a spade to find out how hard it is to dig in that soil. You might also bring several varieties of seed, plant them and see if anything grows.
Next - what wildlife is in the area? elk, deer, rabbits, etc. might love you for this effort - and make short work of your sign... some coyote, wolf or mountain lion lure spread around the sign could keep them at bay but probably would need replacement monthly. Human urine works in a pinch, but others are better.
From your description of the soil, it's probably deficient in nitrogen and organic matter in particular. That means at a minimum, packing in fertilizer and maybe manure / top soil. See what the AG extension says. If you do pack this in, be sure to pack it in dry. The weight difference is incredible.
some plants are picky about their soil and light conditions, others less so. There is also an optimal depth to bury seeds/bulbs. As it's on a hill, worry about getting them deep enough so they don't migrate downhill with the rains. What is the grade of this hill?
An alternative might be clover, that does flower in the spring and would be noticeable as green against that grey/brown background. It also breaks up and conditions soil, adding nutrients/organic matter making it friendly for later things to grow.
If it were me, I'd get a bunch of paper towels and spray them lightly with miracle grow, place in the sun to dry and when still a bit sticky, sprinkle lightly with your seed of choice. then press a similar roll, sheet by sheet on top of it so they stick together. When dry, roll up. When up there, roll out into letters and cover to the correct depth with potting or top soil or manure.
That's a lot to pack up there, so consider a off road vehicle in place of a mountain bike. Alternately, there are pack animals, if you can borrow one. Here's a chart as to their capabilities:
Donkey: 8-10 miles (13-16 km) per day, at best walk 2.5 mph for 10-12 hours a day. can carry 150 lb (68kg), but the heavier the load the slower the walk. speed and distance also depend on terrain.
mules: 3 mph (5km)for 14 hours a day - tougher, stronger and more disease resistant than horses. range not listed.
horses (3 day trip): 4 mph (6.5 km) for up to 8 hours a day, up to 50 miles a day. can carry up to 200 lb (91 kg).
horses (3 week trip): as above but average distance drops to 20 miles a day. You must take rest days.
ponies: no distance or rate given, load based on size but a 14.2 h.h. pony can carry 160 lb (72.5 kg).
Yaks: travel at 1.5 mph (2.5 km)
Sheep: (used in Tibet) can stay healthy in allmost grassless land where yaks and ponies can't find food. max load per sheep is about 25 lb (11.5 kg).
Dogs: (polar regions) a 7 dog team can pull 600 lb (272 kg) and cover 20 miles (32 km) per day. takes substantial skill to learn to handle them.
Camels: 600 lb (272 kg) at 3mph (5km) and will travel 17 miles (27 km) per day. Better speed if travel is at night and rest during the day.
Elephants: have one speed - slow. max load is not given but substantial. Probably a couple of tons, if dragged.
Wives: 45 Ib.. 3mph will travel 15 miles per day. Must be kept muzzled. Speed and distance also depend on how many shops passed.The info on riding, leading and packing this pack animal is as yet unpublished.
Books on how to pack different animals can be found on this list:
This online manual is also very good:
If you pack it in on your back, you could probably get one or two bags each, per trip. It won't go as far as you think it will, so expect many trips. If you do an off road vehicle, you could get in and out fast, caching the soil under a brown tarp. Remember to always repair fences or it will arouse attention.
You could probably do something like my paper towel approach in a night, if you can get all the soil up there, to plant a ton of bulbs by hands is going to take days, if not weeks. Individual seeds, fall in between, but depend on soil conditions. How easily it can be worked.
Hope that helps,
A few other thoughts.
Again, what is the grade of this hill? Will people be viewing it looking up at it or down on it? Probably up. That means your letters need to be thinner and taller to look normal or the message will look short and squat (fat).
A different approach might be to rent a cement mixer or build a larger capacity (5ga bucket) ball mill and mix wildflower mix, soil/manure and maybe some chemical fertilizer - perhaps ammonium nitrate in it. Then use one of those hand push spreaders to distribute it within your margins. The spreaders can be purchased for ~$120-150 or you can rent one. Bulk wildflower seed can be gotten from many places, here is one:
As noted above, getting this stuff up the hill, won't be easy.
On laying out the grid - it's going to be harder than you think, but laying letters out with string at home and having measuring tapes so you can make things end up straight would help. Bring some laser pointers - that would help a LOT! Bottom is this beam, top is this beam, plant some stakes and string. Letter A this wide, break this wide, Letter B this wide, etc.
Wild flower seed is more forgiving to lousy soil, but at a min bring fertilizer and ideally soil. It may not come off at all without soil. Find out what you are working with and go from there.
Great idea, but you really need to think this out, plan and research if it's going to come off.
ps: dress appropriately, both for camouflage, but also because you can get hypothermia in the dessert at night in the middle of summer.