While poking at @ric's QWeb RSS feed, I realised I hadn't actually checked my RSS feeds against the validator, so I did.
Turns out my insistence on using ISO8601 meant my feeds were not properly valid. RSS requires RFC822 date strings, and while many clients will handle ISO8601 it's not to spec, so I have sulked and changed it.
So how Homer has a new release that correctly generates the RSS feeds (and saves them with .rss extension instead of .xml):