[exim] Rewriting from envelope header at transport time

I would like to re-write the from envelope header at transport time.
The headers_rewrite
generic transport option looks like the way to go except that it
doesn't work; and,
presumably the reason for that is that (according to the
documentation) such rewriting
rules do not apply to envelope headers. Then, how do I accomplish
this feat? Thanks
for any input.

         ... doug