I have done some more experimentation and it appears that what I want to be able to do is receive bulletins. If my server sends a message to "BLNWOTA" then it shows up under "Other groups" in the Bulletin board page of aprs.fi.
In UI-View32 the only way I could get the bulletins to appear is to add a budlist filter for the call the bulletins are being sent from. If that is the correct way to do it then that's fine.
If I do this in APRSISCE I see the call the bulletin was from appear in the scrolling window but nothing happens when I click on it. Ideally it would be nice to have bulletins pop up with an audible alert just like messages do, but obviously without the options to reply to them.