I'm on a trip away from my manuals, but the EXT just has a longer loop and a second heater core for the rear. Not sure why they would add a valve to the system, but you certainly should be able to disassemble one of the bad ones and figure out how to force it to stay open.
How do you know it's bad? Low heat in the rear or no heat? Do you have the electric booster pump upgrade installed by the TSB that was issued for customers who complained about low hear in the rear?
MUCH more of a challenge, and far more important to your pocketbook than a cheap hot water valve that you could replace with a straight pipe, would be to identify why you have intermittent engine overheating episodes.
I haven't heard of overheated coolant killing heater system valves, but we've all read about warped heads, and other BAD THINGS that come from lack of good cooling.
What can you share about the frequency of your incidents, and how you're troubleshooting that problem?