Bad news - the ex500 overheated in exactly the same way. suck. Thankfully I had brought my street bike, so I threw the slicks on that and rode it. Had fun in spite of my frustration with the ex500, posting a 2:11.16 on my lap timer in the afternoon.
After lunch I tore the ex500 apart and found that the spark plugs were toasted. No local replacements could be found. And more evidence on the plugs that there is a leak on the engine head, which means that the bike needs to be rebuilt... this is likely the end of my plans for doing the AFM endurance race this year :-(