When running in non-secure boot mode, RMC just needs to start the next stage bootloader. Modify shim (https://github.com/rhinstaller/shim) to ignore secure boot options and just start the 2nd stage bootloader (load_image -> start_image).
Thinking about this again, shim will probably do this out of the box. It might be just a case of verifying if this is the case.
Closing this, we decided that RMC will not support secure boot out of the box.