I’ve just thought of a hack that hasn’t been posted here yet:
File > Export flows. Select all apart from first one. Make sure the new file (containing exported flows) has actually been made - generally this is very quick.
Delete all flows apart from first one, from original project.
Edit master pages at lightning speed.
File > Import flows.
This is tried and tested, and the exporting and importing really doesn’t take more than a few seconds.
Caveats: You’ll lose ALL tweaking you’ve done along the way, so it’s not a step to take lightly…