<snip>

Is there a method for checking beforehand
whether the current backup session is going
to be a base or a meta backup?
This info would be used to prevent - or at least
warn the user to not to use diap-up lines for
huge backup sessions.

Not an issue - it's not the SESSION that is a base or meta backup, it's at
the FILE level.
Only the particular FILE that has changed enough since the base backup will
start a new base.
So it probably won't matter to most users if a few files require a new base.

The only thing I know of that will cause the entire session to be a base
backup, is deleting the /baclient/cache subdirectory.