Dear Tom;
I did a simular project. Sought advice from many pros. Best advise I got was to go ahead and put the wonderboard under the tub, paint it with REDGUARD, paint all your cement board with redguard, then tile up to the tub. If you are going to have a lot of cuts, consider a different layout of the tile. Easier to replace a broken tile if half is not under the tub. Your tub will be heavy especially when full of water and person. Redguard is a water barrier. Good luck

Apart from it being unconventional and not as stable, in most senses it won't be any more water resistant anyway, since sooner or later it joins the board, or the wall, and murphy's law dictates this'll be where you have problems.

Personally, all bathrooms/shower areas etc ought to be lined with rubberised fibreglass and for subfloor it should be cement board (this is how it's done in Australia) but here in BC apparently not, I don't know if you can get this fibreglass stuff here and I think regular flooring in bathrooms is the norm (ok it's semi-moisture resistant, but this fibre cement stuff can live under water with no ill effects)

Personally I think that approach is getting the cart before the horse and could present some legal problems.

In the normal course of events the tub is installed before the wallboard goes in and the finished floor material is generally one of the last steps of the project.

If you install the floor before installing the tub you are confronted with two very serious legal matters, The Law of Gravity and Murphy's Law. From the moment the flooring is put down the Law of Gravity dictates that anything that gets dropped will ultimately land on your new flooring material and Murphy's Law dictates if something can go wrong it will go wrong, ergo cracked tiles or marble, torn vinyl, permanent stains etc and even if you do put the floor down first you must still caulk the joint where the tub meets the floor or water would seep under that edge and form mold under the tub.

One more thing to consider, is the bottom portion on the skirting. There are usually scuff marks and imperfections you wouldn't want exposed. Also the tile could help keep the skirting from moving outward.