Great idea but something to consider: Are you willing to have WGT take 20% of everything the sponsoring player adds to the purse?
If they don't, CC players desiring larger purses will simply set up only 5cr tournaments and add the credits to the purse. That will lower the amount WGT currently gets in games designed with higher entry fees to raise purses.
I'm sure some kind of compromise on the percentage WGT would take on the sponsor-added amount could be worked out but it is something to be addressed.