There is a complex way you could do the hack you'd like to do. For $50 you could buy the Cable
USB/ANT+ bridge and use it to select which Bluetooth and/or ANT+ feeds you want to use for speed, cadence, rpm, power etc. Then all you need is a source of constant RPM via Bluetooth. That would be easy enough to do with some simple software on a laptop, or on something like an Arduino board.
A lot of Zwift YouTuber/blogger/podcasters have ways of doing this - they routinely create "ghost riders" to test things out like which cadence will go up hill faster or why changing the trainer difficulty doesn't impact speed. You could post on DCrainmaker or GPllama and ask how they do that.