We like the idea, though there are a few things that need to be considered:

1) we want to keep a simple user interface and not confuse other miners; this can be solved for sure, but needs to be done carefully
2) the hashrate can only be sent on an average basis. For example, if a user wants to point 1 MH/s to a pool and our smallest miner is 24 MH/s then we would have to point 24 MH/s to this pool for one hour. This is not ideal.
3) if the pool is crappy, or there are some technical problems we would see high reject rates and unhappy faces.

Probably we would require that mining on your own pool happens at least for 24 hours. Furthermore, we would probably require that the maintenance fee is paid from the user balance. In turn the hashrate would be fully used for mining coins.

Technically, it can be done for sure in a reasonable time frame. This would be an advanced feature which only experienced users should use.

Please let me know if you have any comments.