I managed to link it by following what @The_other_Rare says in post #2 above (first page).
I have the same email address for both Steam and Origin accounts, not sure if that's important. When I entered the email for the "new" Origin account, it found my existing one and that was it. I may have misunderstood before, I'm not sure (don't remember).
Side-note:
But there's something funny going on with Origin's Filter settings. It shows 1 hidden game, and 1 Steam game, but none of them actually shows up even if I clear or select said filters.
I only had 2 games bought via Steam, one of which I refunded. The other one I did not refund. Both gone now :P It works from Steam and recognize my Origin account, so no biggie I guess.
EDIT: Nope, the refunded Steam is still there. Just needed to activate both filters at once (Steam + hidden games). Not sure if it complicates matter that I re-bought it later on Origin? Btw that version is also there anyways (not hidden etc), and works. Idk. Hours played is the same on both, probably since it's the same title. It's a bit messy. Or actually it's not, or shouldn't be. But it is. But sure I'll just continue having the refunded Steam version as "hidden", not really a problem. Just odd.