Timing a sidevalve engine is no different to ohv engines, equal lift at TDC as well. The first flat twin BMW from 1923 had one piece cylinder and head too, with screwed in plugs over the valves . So you´d machine seats from there certainly, I did that a while ago on a Terrot 350 with the Hunger valve boring head with automatic feed cutting the 45 degrees seats.
The Panhard engine was quite unknown to me in all details. But I can see machining the valve seat recesses and machining the seats in there is not a big headache once they do this job at the factory before shrinking in the cylinder liner from below - gives acces for a decent mandrel and countersink despite angled valves deep below and head and cylinder unit one piece. Even so, I would not want to operate these Panhards, their designers had christian names all Phils possibly . . . .
Vic