My only issue with this is some of the people might not be there. They might have forgotten their accounts, left reddit or might be literally dead. I have definitely made some posts with some alts that I no longer remember the password to.
If we are going to require permission (which I am not for, for the record. I have a different philosophical view) then I would prefer it be an opt out type of thing. Meaning message the account, say that you are planning on reposting their post to Lemmy in the next week unless they respond and say they would rather not. That’s a win, win, win. Not only will you get a better repost rate (because let’s face it even if people don’t care, some people won’t bother answering their messages), you still give them the opportunity to say no and on top of that you get to introduce them to Lemmy!