Harry, a few points to make:
1. No doubt most propositions are losing ones - period.
2. There are a slim breed of player that probably could show a profit against the vig - but never have any money due to other leaks. You could possibly have a great investment there, but I imagine trust issues crop up with a player with a sickness for other -EV gambling...
3. There were murmurs of staked players being used as chip horses - told to make steady progress in the tourny and then dump any time the guvnor needs them...
4. Whilst someone might have no chance to overcome the odds in the long run, there are worse short term punts out there. The numbers don't particularly matter, if someone has been running well and decides to take a "$100 shot at $10k" for example, they will probably be able to pick a better horse than the 100/1 equivalent you would need to find at Newmarket to return the same.
Of course it's always tough to find someone whom you think represents positive equity especially when the volatility is so great but of course some of them do exist. However I would suggest that your best chance is to go for younger newer players to the scene as the older more established ones are probably already ties up to longer term deals by others.
The integrity issue has always been a major concern.
I recall staking one player that asked me to put him into The Commerce $10k WPT event a few years ago and he wanted 50%. I told him it was way too much and that 30% was a fairer mark especially as I did not share his optimism about the field being that weak. However I still backed him pure and simply becasue he made it clear he would not soft play against me if we ended up on the same table. Without that assurance he would have had ZERO chance of getting backing.