I always thought it was the other way around, which is why HAS has that adapter to feed video from the HAS to the VGA port of the OSSC?2 - There have been reports that e.g. Taito F3 does not with with H/V sync on the VGA port, but works with C-SYNC on the Scart input.
I really need to get hold of an OSSC for some testing purposes.