I did find some searching... It looks like our best bet on this is to take a shot with stage-makeup style skin dyes... This is the kind you can put in an airbrush for fake tattoos (like at fairs/carnivals and stuff), or you can also rub it on with sponges, brushes, or fingertips. A couple drops will spread over a large area.
Check this out:
http://shop.temptu.com/p-33-sb-foundations.aspx
Look at the different shades... I would say the the 010: Cappuccino or the 011: Dark Cocoa would be good for most mid-brown-colored guys' hair, but really dark-haired guys might want to try the 012: Espresso.
I found the 011: Dark Cocoa color on eBay for $18 instead of the steeper $25 that the retail site wants. I am ordering one now, I will post pics for you guys if anything good comes of it. Then we will maybe look for a cheaper alternative.
One other option I've wondered about would be Henna ink; It is dark brown, nontoxic, and lasts for a whopping 1-3 weeks... But how would it look on a pale white scalp? Would it look orange

That would be horrid! But I will be your guinea pig, so to speak. I found some
here on eBay, and I will order a small tube to try it, in a small area first, then with more coverage if it works well enough.
If one of these options works, it could be a great temporary solution for the whole mid-scalp-thinning problem until my savings CD matures in 10 months and I buy a $10,000 FUE hair transplant.
And while you're waiting, you guys could try and scout out some other types/brands of skin ink; I found a thread
here, in a forum for stage-actors and models and tattoo artists, that says that alcohol-based skin-ink lasts the longest. You could search for something like that, too.
Wish me luck, and check back over the next week or so.