Trunk is indeed always trunk. But trunk can have different versions (today: 3.1.1 became 3.3.1)
Stable is a moving target (went in short time from 3.0.0 to 3.0.2 to 3.0.4).
Fixes also moves.
Yes, that's why having those details in a config file can be beneficial: the user can make changes as soon as an announcement is published (like today) or download an update from your site/vcs.
The best I can think of is getting the version info online, making it dynamic.
But care must be taken: users may have their own custom config.
The bonus would be for a small single-file dowload or leting each user update locally just the few details that change when a new release comes (version numbers, svn endpoints, etc.)
Right now, my Win10 machine produces 18 different binaries in a single compilation run. But testing them is not as easy as a single click !
That doesn't change with my proposal. You simply get the version and where-from details from (yet another?) config file.
As I said I just used fpcupdeluxe once and it's not installed in this computer so I may be missing something here; if so, I'm sorry.
edited: typos