So this lets an SME Server on a business-class connection forward SMTP to a server on a residential-class connection with a blocked port 25? I guess there's no free DNS service (everydns, zoneedit?) that does that?
I looked at your rpm...It uses the templates-custom directory to overwrite 2 files, which I think is a no-no. Just a suggestion, but if you'd perhaps like to better integrate this feature into SME, you could inquire on the smeserver sourceforge mailing list whether a patch for this would be accepted.
Seems to me like it would be more appropriate to modify the 'Hostnames and Addresses' panel, to allow you to also specify the port when setting the mail.domain.com to a remote address. That's basically what your panel does, right?