I finally figured how to get the Haloscan comments from my old Radio Userland weblog imported into Movable Type without screwing up my weblog. I knew there was a plugin available that's supposed to be able to do this, but since all of my post numbers changed in the migration to MT, I didn't see how the plugin would know what comment belonged to what post. The fact that the plugin is listed as a 0.1 beta didn't inspire any confidence either. This is the process I ended up using:
Copied comment data from Haloscan website, one comment at a time, field by field, into MS Access database table.
Wrote and ran Radio macro to dump list of old post numbers and post date/times.
Imported Radio list into Excel, inserted new column and filled it with new MT post numbers, then imported Excel list into second MS Access database table. Linked both MS Access tables with a query.
Wrote MS Access VB macro to read query records and output text file suitable for import by MT.
Exported a backup copy of the MT mySQL database (in case something went horribly wrong during import).
Uploaded the text file, imported it into MT, then rebuilt entire weblog.
61 old comments have been successfully imported and added to the MT weblog.