I don't propose to psychoanalyze the boards of directors of IDPA, IPSC, and USPSA, but do have a suggestion:
I expect the match director for any routine club shoot will either give you an exemption and let you shoot your 7.62 or round up a loaner 9mm for you to use. I know I would, in fact I have. We had a guy shooting IDPA with a 6" revolver, and I have seen Makarovs in use. There is the guy shooting the 5.7 but I really do not approve of him because I know he owns a .40 and could abide by the rules if he had to. But the present match director does not make him, so he pops away with his smallbore.
By the way, what kind of 7.62x25 do you have? Tokarev or CZ52, I imagine; a long time since I have seen a Broomhandle in use.
Neither has the safety and ergonomics of a more modern gun so you will have to be extra careful, but if the MD goes along like I think he will, you can make a start in the sport.