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?
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.
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.