The Range Extender is an interesting piece of technology. One of the design trade offs is that it can not be enabled over 75% SOC, presumably as a way of preventing overcharging of the batteries, much like how DC fast charge tapers off as you reach the upper part of the battery capacity.
Anyway I found out how to 'force' the REx to start over 75%. In ISTA, it allows you to invoke the REx testing cycle as long as the SOC is <92%, presumably to allow the tech to test the REx engine without having to burn up battery power by either driving around, or running the heater. You can drive around and it works pretty normally, though it's a bit funny to see marker above the 75% point on the driver display. Anyway, I'm sure there is a good reason for why you can only activate it at 75% or below normally, so this was more an interesting test than anything else-I needed to do my once every two weeks startup of the REx, and it was cold (23F), so not a very good day, but whatever.