Good point - I think there's often a reminder to creators to specify permission to remix and usually a link to the big list on Fanlore, in the initial starter posts. I think doing podfic for a primary fic is pretty challenging for both parties, as it means the writer has to have the fic finalized well before the deadline so the podficcer can then record and edit it in time - and unlike in the Podtogether challenge, this would be a longfic. The podficcer has to be able to record and edit a relatively long podfic in a fairly limited time-period - so that would certainly rule me out as I take ages to edit! I have seen it achieved, but not commonly. It's commoner for there to be a primary work and for the podficcer to locate a story that complements the primary (but is already complete and posted somewhere) and to podfic that. Like, I once did a Spacepirates AU longfic as a primary, and one complement-maker podficced 2 other shorter Spacepiratey fics by a different author. It worked really well.