Ok, I figured it out.

Or rather, I got it to work, not sure how. If you're curious; the RS-232 cables for the MSC-400 include a null modem cross, so adding an adapter isn't necessary and in fact won't work at all (I got this confirmed by someone at URC).

I'm not sure what I did wrong the first time around, because I first tried without the adapter (then added the adapter because I mistakenly thought I needed it). Anyhow, I'm just glad it's working now. The IR emitters are nice and all, but plugging stuff in the back is better :-)