I once did a similar thing for my wife's granny! She was arthritic and a 4" step would have been impossible. We built the granny flat (at my father-in-laws)with this in mind and shuttered the screed for the tray area leaving a recess of about 70mm below screed for the tray to sit in.
The trap and a length of waste pipe were assembled onto the tray and passed through the outside wall as the tray was lowered into position. Tray bedded onto sand/cement.
The tray was a special 'flat' type with a minimal lip at the front edge for access but the tray had a high, thin upstand on the other three edges. These upstands were let into the plaster so that the tiling came over them and weathered it all perfectly.
The end job was a flat shower tray flush with the floor of the bathroom (except for the minimal lip) and it looked good. Of course with the trap and waste bedded into sand/cement it could not be allowed to leak, nor was there any way of getting at the underside but in my experience most trays cannot be 'got at' underneath once flooring etc is laid.
The trap had the removeable grating from above and the waste had an access cap immediately outside the wall.
All in all a good job!