Yes, it would be much better for Damon to get more ABs with runners on, as opposed to DAVID ORTIZ. Excellent plan. Sounds like you're really just desperately searching for a reason to bump Bellhorn down. Millar could use a season off, although I wouldn't be opposed to him PHing for Ortiz in a late inning situation against a LHP. Then McCarty comes in for D. My "shakeup" lineup would look like this.
Damon - CF
Bellhorn - 2B
Ortiz - 1B
Manny - LF
Nixon - RF
Varitek - C
Youkilis - 3B
Nomar - DH
Reese - SS
Nomar may need some time lower in the order to get his swing back, and I'd rather not have him in the field while we wait for his rhythm to return. Let him find his swing (at every 1st pitch), then put him in the field again once his bat cancels out the errors. I think Nixon deserves a few more ABs, and we need to try SOMETHING with Varitek to get him hitting again. Moving him in the lineup is as good an idea as any, IMO. Moving him down would be bad, as you have to either move Youk too high, or not move Nomar down far enough. This leaves Ortiz at 1B, which isn't great, but is probably better than Millar (not bat), McCarty (ditto), or Bellhorn (he's better at 2B than 3B, and he hasn't seen 1B yet this year). Plus you get Millar off the bench to PH, and McCarty for D. They can both play a couple of positions if necessary. Just plug Mueller into Youk's slot upon his return, since he seems to like the 7/8 hole anyway.