However, before I take the cap off the cast iron, I'd better ask...I'm
guessing that a standard threaded PVC coupling isn't going to fit the
threads on the cast iron (at least not with any type of effective
seal). What is the preferred method of making this type of junction? I
can use a rubber coupling again, although I don't have enough length
of iron pipe to cut it, so it would be over the large end where the
cap sits, and I'm not sure if a rubber coupling would have enough room
to seat well.

Replace enough iron with plastic until all you have left is simple pipe-to-
pipe connections, which will connect with the rubber couplings. The iron is
likely old and going to make more trouble, so the less of it the better.