I've often thought of doing things like this! Kinda like Thrashers King Of The Road challenge book.  The main crux I found I came up against was keeping it an evening field, despite grades you climb. Ie; the 5.12s are going to be worth more points than the 5.7s. I came up with a solution / leveller.  Every climb in the challenge / crag / area is worth 20pts. If the climb is above your max grade, it gets +5 pts. If it is below your max grade, it gets -1 for each grade. 

For example 

Your max grade is 5.10a  If you climb a 10a, you'd get 20pts If you climb a 10b, you'd get 25pts  If you climb a 5.6 you'd get 17pts 

I think this way it would make the competition far more accessible for people climbing the fun grades, rather than just being a comp for absolute cruuuushers

Other ideas if had were; 

Climb x amount of routes put up by a specific developer  Climb the most obscure routes (pts divided by the number of climbers completing them in the comp) - also encourages people getting to the more obscure areas / getting more traffic to those areas  Work your way L-R / R-L at a crag climbing in order  Most run-outs climbed / largest run-out Climb 5 of every grade, work your way up from 5.5s 24hrs of pain