@Roberto: The TCZs are the main power of this TC linux. A basic (Tiny)Core ONLY is as useless as a WinXP with just notepad. Good just for didactical purpose only. Porting TC to use musl libc is near useless if you want speed and size gains. What do we talk here about, 1-3 seconds boot speed, few KB gain size?
The power of musl libc is for static linked of MANY small apps, so we talk about TCZs. And of course security of a small musl vs. glibc.
Unfortunatly most recent versions of the apps make use of gnu libc, and libc+. (firefox, libreoffice, vlc, whatever), as you said.
I do not see the point of a small head/"core" with musl-libc and the a fat "body" with gnu-libc.But in the end, if you "make" a TC with musl, then you get an "Alpine"
You compile the free kernel (whatever version you want), and "borrow" the free TC scripts already made for you by TC team (tune-up them a little if you want). What else do you need to "build" it as you like? No need to answer me.
PS: tinycore= kernel + core.gz (modules.gz + root.gz). So your main target should be root.gz. And inside are tc scripts and busybox + few static-linked small progs. Where do you expect to get speed/size gain from? Q.E.D.