Call off the hounds! In attempting to determine was up with the (apparent) linking problem, I thought to update and rebuild my world. I had been building in /usr/7.0/src rather than /usr/src. Moving to /usr/src seems like it has fixed something.
Mergemaster was noisier this time around, and I finally saw the "FreeBSD version not supported" error when compiling the beta drivers. Now, though, everything builds, installs, loads and runs. I've never been more delighted to see the nVidia splash screen.
Looking back at it, though, I think that everything stemmed from me not using the -m option to mergemaster. I guess I expected it to automagically determine from whence my sources came. Moving them to /usr/src and rebuilding accomplished the same thing as mergemaster -m /usr/7.0/src (only with a lot more time and headaches).
Thanks for your time, zander. Sorry for wasting it due to my oversight.
P.S - Hope you had a good Thanksgiving.