If the RJM Mastermind sends the MIDI Start command along with MIDI Clock then yes, it would be a good fit for the 45000.
There really isn’t a workaround for the MIDI Start command. The 45000 needs to receive the Start command.
One thing you can do is set up the loop lengths ahead of time without MIDI Clock. If you know you will need a loop that is 4 bars long, you could create a silent loop that is exactly four bars (do this with Quantize enabled). Make the loop while EXT. CLOCK is off on the 45000. Then when ready to sync to the master clock, enable EXT. CLOCK on the 45000 and press PLAY on the master device. If it sends MIDI Start the 45000 will start playing back the silent loop, you can begin recording on it by pressing the REC button, whenever you like.