Id say the least complicated method would be to present sharepoint out with ISA as in the last post, get an SSL cert installed on the listener. Disable unencrypted http access, configure authentication (remember its SSL so basic is an option, since the SSL tunnel will be negotiated before the credentials are passed) and set ISA to pass the credentials back to the internal sharepoint site
Using the ISA you can also restrict the rule to a limited group of people, so if you have execs that should have it externally and canteen staff that shouldnt, then you can limit who is permitted access from the outside while allowing everyone access internally