Most pool (like mine) pay the bonus for Block finders out of their private wallet, currently we dont offer any bonus for BTC (yet) but i will think on it .
But it will be far less then 50%, maybe there will be a bonus for ~0.1 BTC or something like that.
How much people are interested in a offer like this?
p2pool does that now. The code divides the total block reward by 200. It gives 1/200th to the block finder, and splits the remaining 199/200 with the rest. So, assuming a block with 0 transactions in it is worth the 25BTC, the block finder gets 0.125BTC as a reward in addition to whatever reward he gets from submitted shares.