Here are the options I know of so far:
1. The flanged elbow is mounted to a 1"x4" cross piece with screws that I can get to through the hole in the marble. I could break through the wall from the other side to allow sliding a thicker shim behind the elbow. Other than the work of breaking and repairing plaster, the only downside is that the nipple would assume a slight downward tilt.
2. Make up a nipple with two male adapters and a short length of 1/2" hard copper tubing. I've watched the plumbers do the soldering, but never done it myself. Looks easy. Downside is possibly breaking the solder joint while tightening. I've no idea how strong these soldered joints are in twisting.
Which options is best? Anything I've missed?