Figured out the issue (more or less).
First, accelerant wouldn't fire after the engine hit the no run wall. While not stellar, compression is still high enough to fire. Spark was still good (present and in time). Fuel physically present. That leaves air flow.
More specifically, controlled air flow. What I had failed to catch previously is the rear portion of the carb cooling while running. My end of the shop runs 45-50 degrees this time of the year. Humidity varies from 40-60%. Reasonable conditions for ice clogs in the air inlets. I have checked the crankcase breather lines (which should counteract icing by warming the airbox). Air travels freely. Also, did another round of checks for intake air leaks. Sealed tight. Being a US model, this one wasn't equipped with a carb heater from the factory.
To confirm the icing theory, I used a heat gun on its lowest setting to keep the rear of the carb warm while running. Had no issues when I did that. Obviously that's not exactly practical while riding. What did seem to help was switching to the recommended cold weather spark plugs. While it didn't stop the icing outright, it did prolong the runtime significantly. As so far I've only been bench testing, I have to assume adding airflow over the head will help keep the carb sufficiently warm (though it's no guarantee).
I still don't love the compression. If it were mine, I'd probably throw new rings in it. However, it's a client's that just wants it in reasonable shape to sell off. Given a limited budget to fall in and the remaining fault being environmental, think I'm calling it on this one.
Really appreciate the input. Always helps to have folks to bounce ideas with.