> "@example.com/abc" is one (not particularly nice) way of doing it, but I'm struggling to think of any way that would work nicely.
The obvious 'let's use email instead of that @you thing' would make implementation really non obvious, because it dramatically changes the @ role from the current usage: @ means 'adressed to', not 'residing at' like email/xmpp.
I'd reverse the order, which would read much better and follow known ordering conventions (FQDNs, email...):
email tag syntax inspired: @abc%example.com. That would be @marcoarment%twitter.com and @marco%app.net.
FQDN inspired: @abc.example.com looks much better. That would be @marcoarment.twitter.com and @marco.app.net. It would follow the convention that a non qualified name belongs to the local net. I don't know how that could possibly extend to actually integrate with DNS itself, so that a something-record @marco.marco.org. would resolve to the service handling marco's status stream, MX style.