Your idea sounds like it is just going to end up giving her scale rot instead. You don't want any sort of wet substrate.
If you are reluctant to move the snake out of the 75g, I would suggest covering the screen top with aluminum foil tape (found at home depot, hardware stores). And just misting twice a day while she is in shed. Also, you could make her a humid hide. Don't change one of her existing hides into one, simply make a new one out of a Tupperware container and some sphagnum moss.
Obviously your best solution would be to move her to a different enclosure all together. A 75g is too big for even the biggest ball python.
I would not use perlite as a substrate. That is mainly used in egg boxed to achieve a 99% humidity environment. It is not meant to be used with an acutal snake moving around.
Also, what kind of hygrometer do you have.
As long as you can keep the humidity above 60% for the shed, you should be fine honestly.
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