My problem is that there isn't enough space between the drawer and the existing plumbing:
I can't use a 90˚ elbow like the similar installation I linked to above--it won't fit in the 1.75" space between the MPT nipple and the drawer. The OD of the 1.25" tubing is quite a bit smaller than the OD of the 1.5" MPT nipple. In the past, I just used the nut from a 1.5" tailpiece, made a monster gasket out of automotive drain cleaner, and shoved a 90˚ 1.25" tube elbow (part of the IKEA tailpiece) into it--but that solution doesn't hold up well to drain cleaner.
The 1.5" steel nipple seems to be held in the hub with plumbers putty. What's the proper way to tackle this? Remove the nipple and solder smaller one into the cast-iron hub?