I guess Open Firmware is the canonical answer. This of course compiles for x86. But it's massive and sprawling and not really designed for "can be easily grasped while leaving mental room to actually learn something" but rather for wide-spectrum support.
--
You of course already know about ColorForth (confident guess), which of course fits the bill here. Not quite "build an OS" and more "finished thing", but certainly hits the mark of "rapidly bring up new hardware functionality".
I found http://forthos.org/drive.html a little while ago but completely failed to get it going in QEMU (the CD image boots GRUB, but promptly hangs on loading). I haven't yet tested it with any other emulator, and haven't fed it to any actual hardware yet either.
--
You of course already know about ColorForth (confident guess), which of course fits the bill here. Not quite "build an OS" and more "finished thing", but certainly hits the mark of "rapidly bring up new hardware functionality".
--
I just found 4os (article: https://news.ycombinator.com/item?id=12709802)
--
I found http://forthos.org/drive.html a little while ago but completely failed to get it going in QEMU (the CD image boots GRUB, but promptly hangs on loading). I haven't yet tested it with any other emulator, and haven't fed it to any actual hardware yet either.
HN article: https://news.ycombinator.com/item?id=2973134
--
There's also gfxboot, SuSE's approach to bootloader management.
This is a scary pile of assembly language (https://github.com/openSUSE/gfxboot/blob/master/bincode.asm - warning: 16k lines, large webpage) that parses an equally scary script grammar (see .bc files in https://github.com/openSUSE/gfxboot/blob/master/themes/openS...) that is heavily inspired by Forth (the stack/RPN grammar is right there) but also reminds me of Tcl as well (it uses a { } block syntax).
AFAIK this ships on the install media, and I also vaguely recall building it from scratch being very easy (I used SYSLINUX to bootstrap it).
--
This isn't quite an OS-dev thing, but I think it's fun: http://phrack.org/issues/53/9.html