It is still better to get the nipple out. The following will work with very little access room.
The easiest way to get it the nipple out is to cut it out. Make a couple careful cuts about 1/2" apart from each other through the nipple at the top using a sawsall. Tap out this 1/2" cut section of nipple. Then you can squeeze (collapse) the remaining nipple a bit with a pipe wrench, and it will come out easily.
The only trick is to avoid cutting into the female threads. However, so long as you make your cuts near the top of the nipple, you do have a little room for error (since it is a drain, a little damage to threads on the top should not result in a leak).
I did the above recently on a galvanized nipple in very bad shape. After removing it, the remaining female threads were in great shape, and accepted a male PCV thread with no trouble (or leaks).