I would like to split the output off of one channel of my preamp, with
one side having inverted polarity so I can capture/mix an M/S
recording without using a matrix. I would make a Y-cable (XLRF -
2xTRS with one having pins 2 & 3 cross-wired). Should I be concerned
about the difference in impedance that the output of the preamp will
be seeing by splitting this way? I know that you can burn out a power
amp by not paying attention to the load that it is driving, and I am
wondering if a preamp might have the same issues. Anything else I
should be worried about if I do this?

I know only the very basics of electronics so forgive the nature of
the question, but I definitely would not want to burn out my pre by
recording a 2 or 3 hour concert this way.