> In Python, a winning combo would be an arbitrary (and explicit) FS
> "dtdcache" that people could use with simple a drop-in import (from a
> third-party module?). 

It's indeed possible to provide that as a third-party module; one would
have to implement an EntityResolver, and applications would have to use
it. If there was a need for such a thing, somebody would have done it
years ago.

> It might be interesting to have read-only, force-write and read-write
> modes. 

Please take a look at catalogs - they are a read-only repository for
external entities (provided those entities have a public identifier,
which the W3C DTDs all have).

> Regarding the std-lib, I believe effective caching hooks for DTDs trump
> implementing in-memory or sqlite/FS. 

So then nothing needs to be done - the hooks have been in place since 
Python 2.0.