Relocate bath/shower drain

About to replace a roman bath with an acrylic shower/bath which has a different drain location (offset 800mm from the line of the discharge piping). Would appreciate advice on whether following makes sense and avoids too much jackhammering into the floor slab...
- place a new pipe on or slightly inset to the existing bath floor slab to connect new drain point to existing discharge (wd have ~2% fall towards old drain);
- place mortar bed for seating new bath over the slab, enclosing the new pipe but leaving a gap for plumber to connect the new drain.

Can this layout work or must there be a trap installed directly under the new drain? And what thickness of concrete should be placed above the new pipe?
Thanks.:)