bitcoind and rollntime support
If so, what would be the max amount that a mining client can increment the ntime value?


bitcoind will accept a block and push it out to the network as long as it is valid. Because of that, it's incredibly lenient. Quoting from bitcoin wiki:


  A timestamp is accepted as valid if it is greater than the median timestamp of previous 11 blocks, and less than the network-adjusted time + 2 hours


It almost never matters. Because bitcoind is a single-user mining program, it's almost always on the same computer as the mining agent. If the mining agent needs to get more work units, it doesn't use up extra bandwidth.