Like yourself I am not a great Addon maker but from
what I have seen of OFP it would be extremely hard.
At any rate you would not get realistic thermal data
from objects since the game does not support this.
There are references in the game code to "IR targets"
though but I leave this to someone with more knowledge
of the game internals to explain what function these
might have, if any.
I too think that the lack of thermal sights is a deficiency
in the game since it makes tank gunnery, for example,
very unrealistic in game. This is a pity since OFP otherwise
gives great scope for the exercise of authentic "tankie
tactics". Thermal sights I have read are "rumoured
to be" a feature of the upcoming OFP 2 [whenever that
gets released].