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1) I wonder whether encode_multipart_formdata() couldn't be
cobbled together more cleanly using the email package (I
haven't tried it, but it's got to have most of what you need).

2) I'm not positive urllib is the best place for it, but I'm
not sure what is.

My suggestion would be to contact the web-sig@python.org. 
It seems like the perfect thing that should be considered in
Python's next generation web support.