Oscar Merry has suggested Lightning Comments as a potential solution for cross-app commenting: using "boostagrams" as a method of sending comments. We like this idea; and the spec is simple enough for us to code it; so, you'll find a "comments" section on every update in Podnews's web version. You can also see those comments in the Fountain app.
Podnews are also, as of today, permanently supporting the socialInteract tag for Twitter (and will probably pull in those comments to the page, too, at some stage).
A new version is up: 1.0.0-beta. This release comes with bug fixes and an exciting feature: WebSub! From now on, Castopod will push your newly published episodes directly to open subscription hubs, meaning that they will get broadcasted instantly after publication.