My original reply seems to have started an interesting thread on the
subject on the http-wg list; I'll let you know how it comes out, but
the opinion that the HTTP Content-Length header may be used for each
part of a multipart/* seems to be well supported.
The meaning of multipart is defined in the MIME specifications.
Content-Length isn't part of the multipart definition.
As such, it's only a comment, and should be ignored.