This is similar to something I've been doing for a while now with google reader. Mainly because I am sick of trying to sync downloaded podcasts between ipod, n800, n95, my desktop and htpc machines.

The solution I use is even simpler if you use google reader and Canola...

1) subscribe to all the podcast feeds you want in gReader (adding a podcast tag or something makes them easier to find later on)
2) on a daily basis I read all my feeds and just star any items I want to read properly later OR podcasts I want to listen to.
3) Subscribe the to the Starred Items feed in Canola and it will automagically only pick up those starred items with a media enclosure (video or audio).

You could go a step further and run this through pipes, but to be honest I normally chunk through 1,000 plus items a day and only get maybe 5 media items coming through that personal filter to Canola.

The really cool thing about this method is that i) it requires no fiddling with pipes, code or individually tagging stuff, ii) you can add or remove items from the feed just by clicking the star (and the iPhone enabled google reader site is awesome on the NiT) and iii) it just works.