Are there any hacks that allow a late 2008 macbook pro unibody to use Airplay Mirroring

I'm not sure why you claim to be "sure there is a way to by pass this". If you know something we don't, then you tell us!

Airplay Mirroring uses Intel Quick Sync Video, a hardware feature of newer Intel CPUs. This consists of CPU support, and a separate ASIC on the motherboard, neither of which your 2008 Macbook has.

If you really needed to do it, my first thought would be to replace your motherboard and CPU with those from a newer Macbook, but generally they're designed specifically for their case, so I don't think this would be feasible.

Slightly more realistically, you could reverse-engineer the Airplay Mirroring program, find the code that uses Intel Quick Sync, and write your own version that works without it, perhaps using the GPU (which would be 5-10 times slower but depending on the CPU and GPU could potentially work).

This would be exceptionally difficult, and of course far more expensive than simply buying a new Mac.

As Ken pointed out, there are hardware limitations behind the machines Apple has enabled Airplay mirroring for.

That said, there's a 3rd party app, AirParrot, that has figured out the Airplay protocol, and can mirror your screen to an AppleTV on pretty much any Mac running Snow Leopard or higher (and there's a PC version for that matter).