Using fatdog as a build and template system I compiled the latest 6.6.9 kernel from kernel.org, so no aufs (or patches).
Directly mounting the devx.sfs and then inserting that mount point as a middle layer in a overlayfs, followed by a chroot to / ... and I could run gcc. Exit the chroot and the loading of the devx.sfs disappears (gcc no longer available).
I did set the overlayfs to record changes in a folder that I remove as part of the exit of chroot (unload of sfs) clean-up, but if instead the folder is left you have a record of what changes were made whilst the sfs was loaded, so for instance any changes under /home that occurred whilst the sfs was loaded could be applied to the main systems /home (selective changes such as creating new files whilst the sfs was loaded being preserved rather than lost after the sfs was unloaded).
In a limited sense it would seem that dynamically loading/unloading sfs's under a overlayfs is relatively trivial. For more complex combinations however and its less viable. Singular (or multiple sfs's being) loaded at the same time, unloaded together would be fine. Loading/unloading sfs's in a more random manner would be much more awkward to manage/code, if not impossible.