Would this need to be a gadget? If https://discourse-mediawiki.wmflabs.org/latest.rss is added to $wgRSSUrlWhitelist then this would be easy to do in a template or wherever with

<rss>https://discourse-mediawiki.wmflabs.org/latest.rss</rss>

And if this were whitelisted on Meta, I think it'd then be useable on global user pages too. :-)

I'm worried about the large number of potential different feeds to process for the wiki, if everyone adds their own unique rss feed on their user page. The extension was probably not made to handle many feeds, which could cause a performance problem.

Good point. If there is a risk of performance problems, maybe we could start with the 2-3 main feeds only, then open the gate to categories/tags (and even there we could proceed upon request). At this stage, only if everything is working fine and there is active interest to enable user feed we would start considering that option.

In any case, let me bring this task to the attention of MediaWiki-extensions-RSS

The extension caches fetched RSS data for a configurable amount of time (default: 1 hour) and reduces parser cache expiry to that amount. The latter seems more problematic but is not really related to the number of different feeds, but rather to the number of pages the extension is used on. For the former, I don't think a few thousand different feeds would be too much of a strain on memcached.

Not that great: https://www.mediawiki.org/wiki/User:Tgr_(WMF)/rss_test

Per discussion with @Samwilson, this is probably not worth the effort; writing a custom extension for exposing Discourse feeds in MediaWiki will have better results. Filed as T215053: Write an extension for displaying Discourse feeds on mediawiki.org.