Maybe outside the scope of this project, but I'm asking anyway, because why not.
vNAS provides U.S. departures at uncontrolled airfields with a squawk code before departure, so that when a controller comes online, everyone is already on a discrete squawk.
Suggest all IFR flights are given the same in the UK.
Even better:
API to check if aircraft is on Hoppie, if yes, send squawk allocation by Hoppie. If no confirmation reply within 5 minutes, revert to message via FSD.