Oh bloody hell. I emailed them… and then I found this:
tro-taiwan.roc.org.uk/dc/cvfor2.htm
So they do need the FCO stamp (for which you need notarization). And if you don’t go in person, you have to give a letter of authorization and a copy of your passport to the person who does go, and the letter of authorization has to be notarized as well.
The funny thing is, I’m working as a POSTDOC at Academia Sinica at the moment, and all I had to show them was the virgin unstamped PhD certificate.