you could use a system like bitcoin (just google it for more infos)
Any user have who has a decent gpu run a client which connect to the server, take his job (10 salts as sample) and give the result back.
Every 10 salts processed (whatver the code is found or not) he gets 1 point: once the points earned are 500 (5000 salt processed alias half full bruteforce) he gest one free log on the server.
The customers can both buy logs or earn logn sharing their resources on the main server.
What are the pros for both?
a) For the server: more calculation power, faster bruteforce which means more profit.
b) for the customers : Be able to process their job quicker then working stand alone and earning free logs when their bf computer is not used.
c) For who has not a supercomputer: Have access to a really powerfull cluster (Obviously paying for it
)
BR
Alex