Migrate D2D UI - Source field missing from dropdown menu when editing field mappings
I found the cause of the problem, and a solution:

I had cloned my old D6 site, then performed the core and CCK upgrade to D7 as described in Drupal's documentation, during which I mistakenly converted the above-mentioned field to a Boolean type when it should have been (based on the old site's field) converted to the List (text) type. I manually deleted the field and recreated it as the proper type, ran the migration, and the nodes were imported as expected.