I originally asked the question Implementing PPPoE half-bridge/ip-passthrough to suit IPSec VPN firewall appliance with Linux a little more than a year ago, which no one was able to answer. The question was eventually automatically deleted by the Community user after a year as an abandoned question.
I have found a much belated solution to this (which I imagine would be quite helpful to anyone else in this situation, as I couldn't find any help on this topic by searching). I'd like to leave the answer for anyone else who might need it.
What should I do in this case? Should I try and get a moderator to undelete the old question, or should I repost the old question and put my self-answer there?
(For posterity, the edited and answered question is here: Implementing PPPoE half-bridge/ip-passthrough to suit IPSec VPN firewall appliance with Linux)