Not a complete answer to your question, just something to think about.
If your whirlpool tub is anything like my brother's, it is made of fiberglass.
The mortar bed is to keep it from moving as well as giving support. Your idea of plastic film will defeat the purpose of keeping it from moving. Fiberglass and movement results in the fiberglass wearing at the contact point. Your idea may void any warranty with the tub as well as cause you problems. If it isn't installed for a long time you may not damage anything.....but maybe you will. Feel like rolling the dice?