Holding down the spacebar does work -- I was trying to navigate using arrow keys or wasd as directional controls, since the input tab had that listed as the default for controls.

I'm an embedded developer, with little experience in PC applications, but tons of experience in getting other types of devices to talk to each other. I've never used ANT+ before, but do have a speed/cadence sensor, a heart rate sensor, an ANT+ dongle for PC and an ANT+ enabled android phone. I'm happy to offer any kind of assistance that might help you add support for any of the above.

I believe the sensor I'm using is a type 4 from this page, where the speed and cadence are combined into a single signal, but if you're adding support it probably makes more sense to start with something newer, which would be the type 1 or 3, where each sensor has it's own channel. https://www.thisisant.com/developer/ant-plus/device-profiles/#523_tab

I figured out the issue with the xbox controller -- the connector I was using is just a charging cable, not an interface cable, so it's no surprise it wasn't working.

I have the previous generation of sensors, where the speed and cadence are combined into a single unit that uses magnets to measure rotation. Here's a link showing what it looks like: https://www.amazon.com/Garmin-Speed-Cadence-Bike-Sensor/dp/B000BFNOT8