I can't think of a way to do this in any neat way. If emailgtw has a (undocumented) feature to configure the subject it listens to, you could work around it with nas scripts, so that's something you can ask from support. Another silly solution would be to have nas repost messages under some custom subject, have them posted to another hub where you have your nas listening to this bogus subject. Then you'd need to have that nas dedicated for that subject, though. It's also very ugly.. but doable, I guess.
I'm sure you could do this also on the SMTP server, for example have it relay messages to your SMS smtp server under certain conditions. But that's not a nice solution either..
-jon