So I'm just curious ... any special reason why stratum nonce2 is 6 bytes on bitminter?
To support sub-pools.
Is this a feature available to all or only for special cases? (ie: like when AsicMiner used bitminter)
This is available for anyone. nonce2 is 6 bytes at BitMinter. You can create a subpool that has a 4 byte nonce2 and have up to 65536 active workers in your pool.
In the future the nonce2 size will be configurable by worker account. I just did this as a temporary fix to make subpools possible before the worker configs are available.