This isn't possible yet. For me, the builtin filtering has been enough. But it's worth investigating what it takes to ask spamassassin for a classification. Could you open an issue at github for this?
> the documentation/instructions could be clearer for non-typical use cases, for instance catch-all emails
Agreed, documentation is in need of improvement. So far I'm often pointing people at https://www.xmox.nl/config/. Searching there typically pops up a config option. But it's not the easiest to find functionality that way. The admin web interface also needs to be made less spartan.
The catchall is possible, by configuring an address "@$yourdomain" with an account.
> because it just seems to work
This is certainly the goal. And I think we'll only get better over time!