A userbox is easy. If you mean showing the feed right on the MediaWiki page, not really (can be done with a gadget but then it would be only visible to users with that gadget enabled, or you’d have to convince the admins to make that gadget always-on).
I mean a gadget showing the actual feed. If someone creates the gadget, convincing the MediaWiki.org admins wouldn’t be that difficult, and that is the site where most potential users of this feature would be found.
Another thought is, how would this gadget behave in Meta combined with Extension:GlobalUserPage, when User pages are mirrored in other sites?
Including content from an external app is a script injection risk, so it would have to parse the feed and build the HTML on its own. Not terribly hard but not a simple task either.
Alternatively, we have a bunch of RSS reader extensions; not sure if any of them are high quality though.
I’m not sure I see the use case for this. Just having a link to the user’s activity here would be sufficient?
Once we have more tags, it might be cool to have a feed of a tag’s recent activity. For example, if there’s lots of discussion about a particular extension, it could be tagged as such here, and then the feed could be included on the extension’s mediawiki.org page.
What was a pseudotechnical question has turned into a possible tech project. I have created a Phabricator task for it at https://phabricator.wikimedia.org/T185075.
Let’s continue there.