That's an interesting take and certainly would feel like the players had accomplished something, esp. Combined with silver vulnerability.
Thing about other suggestions for DR, resistance, and temporary hit points is they generally do the same thing - the monster buys time to do more cool stuff. In the spirit of speeding up combat I want to cut to the cool stuff without needing the monster to last an extra round or two, or more if encountered in packs or without silver weapons (not to mention non-weapon users getting screwed).
Does that make sense?
That does make sense, but your initial post said you wanted something that "maintained the feel of regeneration" and therefore you have variations of other healing as options. Anything else might make a replacement but not necessarily give the same feel as a healing effect.
If you just want a replacement, you could go the route of the shifter mechanic in that when bloodied, they can gain bonus +2 damage, or +1 AC and speed.
You could also make the regen only work as a recharge power so that there is only a chance of it actually kicking in on any given round.
You could make healing (or gaining temp hp) cost an action (i.e. a Move action) so that dazed/stunned/poorly positioned/prone/etc couldn't necessarily do it without giving up something (or take it even further and make it a standard action)
You could make their regeneration active when NOT bloodied but then stops working when bloodied (the pre-bloodied portion of combat tends to go quicker than the post-bloodied portion if your party uses hard hitting daily/encounters during the first few rounds rather than towards the end when it tends to be at-wills trying to counter-act the efefcts of regen)
You could just give it in fluff but no mechanical effects "Your sword slices through his skin, but even then it seems like the wound might close itself if you do not bring down the beast quickly!"
You could make the regen no longer a per round thing but a per short-rest thing, thus if they leave any of the werecreatures bloodied (or it escapes, etc) it can heal itself up after a single short rest allowing the PCs to recognize the creature as the one they just fought but fully healed...