Now you tell me! There is in fact a way to automatically ping Topic Exchange from within Movable Type. Thanks to an old Ben Hammersley post, I found out that the category attributes page has a box to enter TrackBack URIs to automatically ping.
So after all my work over the last week, I've managed to set up MT so it:
1) adds ENT data automatically to my RSS file, which allows KC to aggregate my posts; and
2) auto-ping TE, which allows it to aggregate my posts. Whew!
Right that really is the end of my topic-mapping shenanigans for a while... I've done my bit for the community. I'm off to read a biography of Hemingway (yes, really).