Seems like the ETH work on Oberon etc did that. The students kept learning by improving on or porting both the compilers and OS's. A2 Bluebottle is a fast, usable OS as a result. Barely usable given no ecosystem and few features. Usable, though, in a safe, simple, GC-ed language. :)

Far as C compilers, would this one previously posted fit your requirements?

http://c9x.me/compile/

https://news.ycombinator.com/item?id=11555527

It seems to be quite similar to what you describe. Designed to be compact, easy to understand, maybe easy to extend, and useful in practice. I remember liking it more than most of these submissions for those attributes.