Generally when we do a recessed slab sort of handicap shower we give a slight roll to the pan instead of a curb, so there is some height there.You can also make a step down roman tub.The trouble with most recessed and curbless showers is the pan, verses roll on waterproofing methods.You see it would be easy if you could just replace the pan with a roll on, but the roll ons, don't want to stick to the drain so you have a leak area.The Kerdi system has it's own drain, and pan junction..no leaks.I personally don't use the system, but it could be your answer,check into the Schluter site, or around this site there's an explanation of it.