I find myself wondering what the cost of a low-power ground radar capable of interpreting transponder codes, coupled to a small (not much horsepower) computer capable of matching the geo-coords of the txpdr to the assigned runway (or taxiway) and triggering an alarm if the variance is too great

It would not help in the case of intersecting runways when the aircraft
is =at= (or close to) the intersection and lined up for the wrong one.
It would not know that the pilot is intending to take off, rather than
simply pausing for traffic. (this means that there could be many false
alarms or many misses, depending on how the software is set up).

This should all be off-the-shelf technology, shouldn’t be considered a “flight critical” or “safety-of-flight” system (so if it goes down for a few hours only the local information would change)

.... and it may well be down when it's needed. One of the issues with
the LEX incident was that lights were not working properly, and there
was construction on the runway. It shouldn't have mattered (as your
proposed system's down time shouldn't matter) but if the system is
there, and it's relied upon, and it's down, then even though people may
know it's down, it provides a hole in which to bury oneself.

That's why we practice partial panel and such, and why there is a
reaction against all the gee-whiz (moving map, glass) in the cockpit
with some pilots thinking it reduces general pilot competency.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
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