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We are in the process of doing a bathroom remodel and we noticed that the bathtub we ordered does not have a flange. The bathtub will be installed in an alcove setup (surrounded by 3 walls) and will mainly be used as a shower.
The bathroom is currently stripped to the studs, the floor is not finished (just leveled).
What is best practice to install this sort of flange"less" bathtub?
We have already purchased 1/2in Durock and stone tile that will be used in the bathroom walls around the bathtub.
I was and still am in complete agreement with paperstreet's answer, buy another tub. I've never used this system. These look like a fail waiting to happen. Available from schluter.com, asked about at johnbridge.com calling them field installed flanges.
I hate to give an answer that's not really an answer but I think you need to return that tub. Without a flange there's no way to guarantee that water won't migrate over and around the edge of the tub into your wall where cavity where it will fester, unseen until you have a major problem. If someone has a better, more can-do, solution I'll be the first in line to delete this answer but until then, sometimes you just have to cut your losses:(
If replacing the tub isn’t possible a combination of materials is a good option. Construct a frame to support the tub. Use backer board to cover the the frame as you would a wall cubby. Tape & cover with thin set mortar, roll-on Redguard moisture barrier. Set tub on frame and use an edging trim to hold the backer board and something like what Supply house.com sells, the Pasco inside corners for lining a shower will insure corners are leak proof. Oatey makes an adhesive to insure there is no movement. Consider clear Quad Max window & door sealant that works on metal, pvc, vinyl on this area because it is clear and adheres lock tight to tub and tile. Save calking for other tile wall areas. Any similar combination of materials will insure there isn’t any risk of leaking. 
