Author here. A big thank you to you all for your interest in muxfs! I will try to answer all of your questions as best I can.
One question: do you plan to implement "concatenation" of filesystems, so you can build one very large muxfs system (10s of terabytes) ?
I don't plan to add "concatenation" as you have described, however this can, in theory, be approximated by layering muxfs on top of multiple RAID0s.