Barney—funny, it was Google Reader that I first turned to. I similarly use the share feature to call out the items in my feeds that I want to look at more deeply later on my PC.

The only problem is that it doesn't work for items that haven't appeared in your feeds... Say you stumble upon a reference to a podcast on a blog post somewhere and want to listen but not subscribe.

One interesting possibility is the new feature GR announced recently that lets you add something to your GR shared items using a bookmarklet—the only downside is that the media itself may not be enclosed (b/c what GR grabs isn't necessarily syndicated), unless you marked the MP3 itself, which would have to have been opened to use the bookmarklet. (unless GR is smarter than I give them credit for and actually scans an item for an MP3 link to enclose)

Using my method, you can tag any link to an MP3 in the wild using del.icio.us and run that tag through this pipe, so you can collect stuff from outside your feeds—from anywhere on the web—and pull it into Canola.
Once you configure the pipe with your del.icio.us username and intended tag (the two form fields), it's "fire and forget"—no fiddling with the code of the pipe (unless you're into that kind of thing). And you could configure a delicious bookmarklet that would prefill your tag for you, which would make it super easy, maybe even 1 click or a FF keyword if you really wanted.