@roberto: do you know that one goal in TC is to REMOVE an extension on the fly, without rebooting?
I do not know about unionfs mounting, but with linked squashfs is simple, and is working.
What improvment (seconds in loading speed) do you get and for which (how many) tcz in the same time?
FYI: for my needs (firefox, vlc, ffmpeg, libreoffice), if we talk about big/fat dinosaurus, is about 3-8 seconds loading. Regarding "productivity", for me, if I would spend 3-5 hours one-off to design/test/change/test etc, then is not worth it; I need 3 times/day mounting x 30 days/month x 12 months x 3 years to recover "my investment" in this project. IMMV.