Right now, we only provide a very low-level interface to all the functionality available in Xvnc, via the /vsmagent/xserver_args setting. This merely appends extra arguments to Xvnc's command line, so it's up to the user to figure out what arguments there are and how to use them. There is also the risk of interfering with hidden arguments added by vsmagent, causing unknown issues.
A more user-friendly approach would be to provide explicit settings for the various wanted functionality. E.g., the session lifetime settings.
This would also be more future-proof, as we could more easily change the mechanism.
When doing this, we would likely also need some migration step in tl-setup.