How do they do a shower stall when building on concrete slab? Normally the trap for the floor level drain would be under the house in the basement or crawl space.
Just put a box of sand where the trap goes - about 1/2" under finish grade. When you pour , finish the concrete right over it then come back later - take a hammer and bust out the thin congrete - attach the trap to the end of your pipe which is under the box of sand with the end taped shut - leave extra and cut to fit. Proper measurements are reasonably important but you will have enough room to work. Make the box large enough to not give you trouble - maybe 12 x 12.
You could also have the trap on already and just leave the 2" pipe sticking up under the surface in the box of sand if you lay out carefully.