Seems like we were overthinking this one. Editing the translation for js.filters.with_category
will accomplish this.
The description is taken from the category’s description, which can be edited as usual.
<head>
<title>Discourse Ruby Topics & Discussions - Site Name</title>
<meta name="description" content="Discussion about the Ruby programming language.">
...
@tolkin See if this is enough for what you need.