cattledog
Haven't any money so I will beg........ 
Beg for as long as you like, but as I said
"Unlikely I expect, as it has never been added in for the last 16 years."
This is probably a feature that is not used by many people, so therefore the need to add it to SME server was never considered important or could justify the time needed to create the code, & that is apart from the practicality & reality of SME server actually supporting that protocol etc.
It might be something that is not practical to implement, & that is why it has not been done.
The workaround provided works fine, so if you really want to do it, there is an existing answer.
Otherwise use a different ISP who supports protocols that SME server supports (with modem in bridged mode & SME acting as router/login client/DHCP/gateway/firewall etc).
The best approach is to open a bug, request a new feature be added (NFR), & see what the response is from developers.
I have recollections that it (ipoe) has already been raised, so search bugzilla first.