This feature could go hand in hand with the osu!direct feature for the supporters. With this being, have a new tab and/or something like that, that opens another screen in the osu!direct, kinda like forwarding to a page. On there, are a list of mappers that you're subscribed to.
Of course, a subscription should be done on the forums, and with that, have it where the game reads the list, and displays the username. You could include maybe something like a graphic banner for the user, that the user creates and/or one that's automatically generated like the osu! online list, or the stats bar you can find on the forums.
It's a good request, I'm surely in for support.