If not, could someone make one? For example, jwz, could you add it to your cheesegrater?
The screen to scrape is probably http://www.ibiblio.org/Dave/this-week.html, which changes every week (and during the week as new cartoons are added). 'Twould be nice to include both the caption (not all cartoons have captions) and a link to the image.
If no one speaks up, I could try my hand at this, but I've never worked with RSS before. (Oh, and I'm not sure what the results are of the feed getting longer during the week and then going back to one entry when the next week starts -- whether the previous week, or all previously read entries should be cached, for example.)
Oh, and FWIW, I talked to David Farley who draws Doctor Fun, and he said that if an RSS feed can be set up externally, that would be fine with him (and he'd add it to his list of mirrors), but that as far as he knew, ibiblio doesn't provide one themselves.