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How about the advertised exhaust wrap for the converter, and possibly a temp probe that would read the temp of the outer converter heat shield. Then you could drive normally where you know the cat is lit and get a reliable reading for the external temp of the heat shield.
Maybe a new tactic for operation while still retaining high efficiency.
I know an inexpensive laser thermometer will read up to 500 degrees, which makes it a very useful tool for auto diagnosis.
An analogy is your radiator, both it and the cat are designed for the worst case scenario which is climbing a steep grade with a fully loaded car at 70+ MPH. In that scenario the exhaust system is probably glowing red from the manifold back to the point where the factory heat shielding ends. I remember climbing a mountain in West Virginia in my AT XB and the engine was running at over 4k RPM for 6 miles to the top of the mountain. If ever the cat was going to melt it would have been on that climb! Likewise the radiator is designed to keep the engine from overheating under the same circumstances.
Both the cooling and cat systems have to be designed for the worst possible scenario, something you will never see at 50 MPG. Hypermiling basically amounts to trading the heat energy normally wasted for useful work moving the vehicle.
If you can find a happy medium, you might be able to keep the cat lit in all but the most severe engine off P&G scenarios. Who knows maybe even all of the scenarios of your technique.
Maybe you get the picture about me, NEVER SURRENDER, NEVER SAY NEVER, there is always a way to make something work. Even at double the mileage of the average XB driver you still have enormous amounts of heat energy wasted. Maybe not two thirds, but still at least 40-50 percent.
With that amount of heat energy available, even though it is much lower than a normal driver, you can still make every effort to focus it on the one component that depends on waste heat to function properly. Grille blocks increase the return coolant temp, Cat wraps increase the internal converter temperature. Heck even DFCO is about the same as engine off, except the engine is pumping fairly cool air through the cat, which would make it cool off much faster than shutting the engine off altogether, so that is a positive and probably a big one.
Both of those will directly (cat wrap) and indirectly (grille block) focus your much lower average wasted heat energy to the one place where it is essential to maintain proper emissions function.
Tell you what, if I was Sentra, I would not surrender quite yet, You may find that you can actually improve both mileage and emissions beyond what you were achieving before you suffered this "catch 22" situation. In doing so you will be way ahead of any other extreme mileage driver who just rationalizes the cost of higher emissions with the statement "I'm producing lower CO, so that is my justification".
I think this may be one of the most important threads I have ever read on this site, and I know this. You are correct to have a high sense of personal responsibility and integrity. To be willing to sacrifice technique for emissions is a high standard followed by few.
It will be very interesting to see if you can come very close to your previous mileage and meet emissions standards. What would be even more fantastic would be to see if you could actually equal your prior average WHILE not sacrificing emissions in the process.
While the design of your emissions system may make that impossible, I would never give up the pursuit, just remember, even the heat energy you are not wasting contributes directly to global warming, so you, by driving more efficiently, are directly heating the atmosphere to a much lesser degree than 99.999999% of every other driver. It's not a rationalization to pollute, just a point to remember.
regards
Mech
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