Why not use Scrypt as intended. Scrypt with default variables has beyond horrible performance on GPUs. Litecoin developers modified it to make it roughly 128x less memory resistant (using only 128KB total).
I'm working on this too and the problem I'm anticipating is that it will take time to verify the hash. The performance I'm seeing running scrypt to require 256MB of memory is a hash time of at least 0.5 seconds . . . . time increasing linearly with memory required . . that's not so bad for mining where you can just have a low difficulty, but creates a problem for clients verifying the block chain - it makes it a slower process, and could start to bite as the block chain gets longer.