Coming in Betaflight 3.4 + You can now turn your Flight Controller (only on f4 of above, f3 is not supported) into a HUD device, meaning if your fc powers your RX, you can use it as a wireless simulator!
Simply goto Betaflight CLI and type the following
Set usb_hid_cdc = on
save
Be sure to turn it back off if you are going flying.