Interesting idea but the revenue split is not worth it.
Why pay you 50% on a sale when someone can generate 100% by spending $8?
The flip side is... you will get a ton of names that will never be sold and you will betaken advantage of.
As far as the donation, that should be coming out of your split, not the guy doing 100% of the work to sell the domain.
Would I consider it?
Make it 80% Domainer, 20% for Registrar and donations comes out of your end.
The other way you can make it work is do a split but the domainer's 80% goes down based on how many names they registered... so for instance...
I register 20 names, $8 reg fee on loan. = $160.
I sell one domain name for $1,000... Sale gets split $800 to me, $200 to you with you covering fees. The $800 is reduced by the $160 I owe for the domains I registered that were not yet paid. Your 20%, call it the opportunity cost.
The split can be altered based on what other tools you provide, such as marketplace, escrow, brokerage, etc.
But no, otherwise I would not give up 50% of my revenue to you for loaning me $8 and 20% to a cause I may not believe in.
And I am assuming with your model the registrar owns the names since you paid for it?