... title pretty much says it.
I think it would be useful to have a checkbox next to each podcast instance to let you specify that you want to keep it regardless of your global podcast settings. In other words, it would not be deleted as part of the normal 'keep last xx podcasts' rotation.
I guess that something to discuss/decide would be whether 'archived' podcasts count against the number of files in a rotation (1,2,3,4,5,10,20,30 or 50), or are additional to it. The latter seems more logical, since otherwise a rotation of 20 with 15 archived would mean that effectively, you are only keeping 5 unarchived files before deleting.