rewrite sender address when recipient is non "local"

[btb]

i have a scenario in which certain email is sent using envelope senders 
that contain host names that are known only on the local lan/network, 
and unknown on the internet.  most mail expressing that characteristic 
stays local, but occasionally, some is legitimately destined for the 
public internet.  to that end, with such mail, i'd like to change the 
sender domain part to @example.com, but only if the recipient domain 
part does not end in example.com [both the sender and recipient domain 
part may be @example.com, @foo.example.com, @bar.foo.example.com, etc].

what is the right method for doing this?  given 
ADDRESS_REWRITING_README, it seem to possibly be a fit for either 
masquerade_domains or smtp_generic_maps, but i'm not certain, and i'm 
not sure how to apply selectively.

-ben