One possible use for this: mark the "str" buffers of PyUnicodeObject instances when demarshalling docstrings from disk; in theory these ought not to change, and can be quite large: the bulk of the memory overhead is stored in a separate allocation from the object, and thus isn't subjected to the ob_refcnt twiddling.

No idea if it's worth it though; the syscall overhead might slow down module import; also, KSM works at the level of 4K pages, and it's not clear that the allocations would line up nicely with pages.

FWIW, various related ideas here:
  http://dmalcolm.livejournal.com/4183.html
Again, no idea if these are worthwhile, this was a brainstorm on my blog, and some of the ideas would involve major surgery to CPython to implement.