David:
You proposed order of signing before encrypting works only if the MSH
takes care of both signatures and encryption.
In the current Messaging spec, the MSH is responsible for signing but
not encryption. Therefore, if you are concerned with persistent encryption
of the payload portion of an ebXML message, the encryption will have to
be performed first. The encrypted payload(s) will then have to be passed to
the MSH for packaging and signing.
Regards,
-Arvola