Thanks, Brent -- I tried this, but still no dice. I didn't copy all of your directives, since there's no relay involved in our case, but I did make sure that the following 3 were enabled...

FEATURE(`allmasquerade')dnl

FEATURE(`masquerade_entire_domain')dnl

FEATURE(`masquerade_envelope')dnl

If sendmail doesn't have any sort of rewriting capability of messages, is there something somewhere outside of sendmail that I can set to get the envelope-from and from: line to change? (Besides changing the hostname...)