The unfortunate thing is that the dude pretty much refuses to do anything with gumroad.
He mentioned like ages ago that he was thinking about it. But it's pretty clear that he has no interest in allowing people to get past rewards if he can't have 100% control over them.
but honestly I'd love it if he'd place his past rewards on something like Gumroad. Like it's super tempting to pledge to him, but the fact that you are screwed out of all of the past rewards regardless, makes it not really worth it imo.