This has come up in the official Garmin users forums a few times with respect to folding bikes as well trainers. Some have removed the speed arm from the cadence sensor by removing the pivot screw that is loosened to adjust it, and then extended the wires so they could mount the speed arm separately from the body of the GSC10. The wires are pretty fine so it takes some care and skill with a soldering iron to accomplish successfully. FYI, GSC10s go for $30 with free shipping on Amazon.
With a Garmin 705 and 800, if the speed sensor isn't sending, the unit reverts to using gps for speed. I'm not sure about other models, but you could inquire in the Garmin forums.
https://forums.garmin.com/forumdisplay.php?f=20