You're right, holds are hard to chase. That's for sheet sure. However, a few things i look at are:
1. What team are they playing on (are they playing with leads in close games?)
2. How secure is their position in the bullpen (not necessarily 8th inning setup guy, but are they being used enough)
3. Track record
Out of the guys you mentioned i like Nieve. Best team listed (Besides Hawksworth for STL), best grip on 8th inning spot, best track record.
However, if Jose Contreras is still avail in your league, scoop him up now. He'll be a holds master for the Phils and may even get the occasional save. He's throwin darts and hittin the upper 90's. Seems to be a real good fit for him.
Better yet, atleast for me, is that he has SP eligibility, so you can max out on Holds/Saves from the SP slot and fill in yet another RP in the RP slot. It's a beautiful thing.
GOod luck and thanks for the help w/ mine