Now it's working. Installing tcpdump (as suggested above) verified that the queries weren't getting to the domain controllers. I also saw lots of traffic back and forth to our DNS servers, so I figured I wasn't specifying the domain controllers correctly in the script.

For the record: Slashes are bad. I'm sure I tried that before, but perhaps something else I've done in the meantime (updating Samba, maybe?) was also necessary.

Eh - the important thing is that it's working, and as is so often the case, the denizens of the Monestary helped.