Indeed, just look at Slush for today 
I just bought an S5 under Hashnest's 'Purchase Miner Converted to Hashrate.' After it's deployed I would like to rent out its hashrate at nicehash instead of pointing it to Antpool, the default. Do you think Hashnest will let me do that?
No. At one point miner link was meant to be released which should allow people to control their power but that hasn't happened yet.
Btw Boyd, I don't think that you would want to point your power at NiceHash. Not at the current avg rate anyway which is around -8% of what you would mine at 100% pool luck. If anything then now is a good time to rent some power on NiceHash to mine a good pool. If you pick the right pool and it hits ~100% pool luck you'll make a plus.