Why not use BOINC as reward system?
Reward mechanism -- BOINC-based;
Transaction verification mechanism -- PoS, for example. (because it doesn't made a lot of calculations)
For verifying rewards I suggest hardcode BOINC projects (for example, WCG, Einstein@Home, Rosetta@Home) using an API.
It like this:
Miner's computer: -- Hi all in the *coin network! I'm a $USER. My boinc-client was sent work and I got N points.
Other computers: -- Yes, API of Project@Home said that you really sent work and you will get M coins. (or "Nothing" if Miner's computer is cheating).
When all coins is mined BOINC-based mechanism will be disabled and *Coin will be 100% PoS.
What you're think?