Thanks, edcho, I actually saw that thread but it's pretty old and outdated. Like I mentioned, the routing is allowed in their timetable (and I've booked it in the past) but is not bookable through either the CO or UA engine. Very strange.

In an additional twist, I can book the ticket as a multi-city itinerary on .bomb but not on CO.com, probably because the first leg is one of those weird CR7/Skywest/CO-but-not-really-CO flights.