Olerud
The Mariners are supposedly about to release Olerud or designate him for assignment. If he'd be willing to come to NY and play part time, I think he'd be a useful addition for the rest of the year. He'd be the best defensive 1B we'd have, and as a lefty, could work in a pseudo-platoon with Phillips, with Piazza going back and forth between 1B and C.
Even with his off year, Olerud is hitting better than Phillips and Zeile, as well as Garcia, who he can replace as the non-Valent lefty off the bench.
EqA this year:
Olerud .268
Zeile .250
Phillips .225
Garcia .236

Olerud had a .278 EqA last year, and .320 the year before.
If we can get him for almost nothing, I think it's a no-brainer. Of course, some team may be willing to give him a starting job, so he might not want to come to play part time.