Is there a way to get the space-usage of the current transient journal on user-session-level?
I.e.; how to identify sessions that cause filling up the transient journal because a commit isn't given?
Not really. The best you can do is to track the number of maint SQL requests (via DBQL) for each session after the last commit (and that will only give you the number of requests, not the number of entries in the TJ).