<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.