This reminds me a bit of Edge Side Includes (ESI), although I realize this is done on the client side more than the server side.

ESI got alot of attention about 10 years ago, but then sort of fell out of favor in the tech media/blogs. Some big companies are still using them like Akamai, and the Varnish HTTP accelerator has some basic support for it.

I always liked the idea of breaking up my page into smaller segments, and then caching each part independently, and assembling the page from the cache. The cache could request only the parts of the page that aren't in cache/expired/uncachable, but otherwise pull everything from a super-fast cache.