This is exact what I am doing today.
But the whole idea was to get rid of the router. Then I could have a small "green" server / router in one unit.
IMHO you should stay with your router..
what you want to achieve needs many (and big) modifications to SME: so, at the end, you'll have a (likely) unstable server and, for sure, an unsupported one.. do you still want to do it?