Looks like QKB and FNL are now active city codes for United's various applications, but only Breckenridge is currently bookable. Schedule appears to be one bus "flight" a day to start: 11:15am - 2:00pm DEN-QKB; 2:25pm - 4:55pm QKB-DEN. Most economy fares (including X) are loaded, with normal connect times on the DEN-QKB leg, and about 60-90 (scheduled) minutes for connections at DEN on the return from QKB.

Reportedly, boarding passes will be issued and bags will be checked at QKB, handled through to United at DEN, and the release says they are "working on" remote security screening, but that possibility seems more remote.

It's certainly something I would try.