I'd be careful about going all out for a protein only type cafe/restaurant. As you say menus are goign to be very bland and boring for the most part. (I'd recommend a look at http://www.proteinpow.com/ if cooking for yourself though).
I go over to the US a lot and generally it's just calorie counts on menus in all restaurants/cafe's in certain states along with a basic macro breakdown and that seems to help everyone. Obviously figures are approximate.
I think if you concentrated on providing macros as accurately as you could it may take off, obviously some healthy dishes are required but a bodybuilder for instance may want to come in with someone who doesn't give a stuff and just wants 'normal foods' you have to cater to both to get enough trade I would imagine. Or just a regular place with the macros and calories would probably do the trick 
If you just opted for protein based recipes I seriously doubt the business would survive long enough as I doubt you'd be able to pull in the required number of regulars.
Saying all that I know nothing about running restaurants, it's just a view from the outside.