Sean, that's a clever idea, but... as others here have mentioned, that heated air setup does zilch for engine warmup. I found this out close to 20 years ago with my '88, and even in sub-zero New England temps with the car parked outside, the presence or absence of the heated air coming from the exhaust manifold "stove" made no difference in engine warmup or gas mileage. Subsequent experience with 2 other 240s confirmed my original observation.

The idea behind warming the intake air was simply to regulate the incoming charge air to within a fairly constant range. The engine controls (injection, ignition, throttle position, lambda system) were primarily driven by exhaust emission regulations, and all represent compromises of sorts which are optimized at a relatively narrow band of conditions. Regulating inlet air temperature helps by reducing one variable from a huge swing (say, -20F to 180F, typical underhood temps) to maybe a quarter of that.

One of the other posters indicated that he liked the idea of a warmer inlet charge because the air was less dense than colder air, so less fuel would be required. Absolutely true. But that also means lower performance... and with 113 hp schlepping around a 3300 pound vehicle, I'd rather opt in favor of performance!

I don't mean to rain on your parade... and again, you have a clever idea. It just is an answer to a "problem" that doesn't exist.
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No one ever went broke underestimating the taste of the American public - H.L. Mencken