Brand stores offer a private space but these are designed for large ISVs etc.
Regarding the issue you describe - auto-connect of a serial port with hot-plug - this can be achieved by requesting a snap store declaration right here in the forum that allows so-called greedy plugs - see Auto-connection request for `arduino-mhall` serial-port for a good historical discussion from another snap publisher requesting this exact same access.
So all you need to do is update this forum post to instead request a greedy plugs declaration for serial-port for your snap and provide some details as to why this is required for your snap. Also I think so far you have not mentioned what the snap is either - that is also required ;). Thanks.