Can't you simply create a new season or title for your podcasts before they start falling off and make an audio reference to this feature when you are at the end of each season? maybe apple would get mad, i dont know, but i thought of this and seems like it should work.
We tried something like that once. We had mixed results.
There appears to be no really clear answers. If itunes just kept working, then we probably would never have stopped to think about it. But the big thing is that we just keep running into technical problems. So we tinker, tinker, tinker ....
At this moment, i think the path is to move all of the older content onto scubbly and embrace the itunes thing about only the most recent 40.
And once we are there, we might think of other things.