I’ve done a few hours of follow up testing:
576p, 480p and 480i via AV2 (PS2) tested 45 mins to one hour each with no issue.
480i and 240p signals via AV1 (PS1) for over an hour no issue.
AV3 (Dreamcast) 480i and 240p over an hour no issue.
While the OSSC was experiencing signal loss due to overheating using 480p on the Dreamcast via AV3 I also tried turning off the console, flipping the VGA switch to 15Khz and booting up a game with no issues after 30 minutes. Rebooting the console and flipping the VGA switch back to 480p caused the issue to return immediately. I repeated this a couple of times and found the results to be consistent.
From what I can tell the issue is isolated to AV3 RGBHV @ 480p.