detaching existing extensions with new systemd extension attach

Hello systemd,

We are looking into systemd system extensions for quick patching our immutable images while we are updating and rolling out the patched immutable image. 

One thing that is stated in the documentation is that before attaching a new extension, all previous extensions are deattached and everything are attached again. We understand that this sequence is needed for overlayfs stacking in case something has changed in the stack with the new extension. 

For our use case, we would like to be able to push multiple patch extensions (ex: couple of days apart) while keeping the service downtime to a minimum. Technically we believe that if the extensions are not overlapping there is no need to detach already attached extensions. There is no built in option in systemd extensions to keep the existing extensions right? Due to how overlayfs works, they temporarly need to be removed and thenbrought up again, during a refresh operation. 

While on the topic, we have investigated other file systems. One we put our attention to is mergerfs, which is a fuse file system to allows "mount points" to be dynamically added/removed without doing a mount/unmount. Is there support in systemd extensions to let system extension work with other file systems than overlayfs?

Thanks

Umut