We have done that in Texas frequently as well but my experience is mostly in areas with no inspections. It looks like this is a new requirement in IRC 2015 code...IRC 506.2.2 Base says to use 4" of sand/gravel base course under the slab BUT you can pour slab on native soil if it is well draining or sandy-gravely Group 1 soil according to United Soil Classification System. We have well draining soil at our site in CO, but I am not sure how it will look 5 feet below grade. We had Group 1 soil classification from our septic test pit supposedly dug to at least 8 or more feet in depth (reported on the engineering diagram for septic system) so maybe we won't have to have 4" of fill. Then maybe I can just use 1.5" of cover or so above the high point of my 3" drain and my ejector basin could be at or just under floor level. I will talk to our basement contractor this afternoon and will see whether we have fill or going on native for the slab. I was thinking I could do this even if we had 4" of fill because I don't see that its prohibited per IPC/IRC, but I will see what my contractor knows about what is done locally in the CO high country since I don't see a direct requirement in either code. The plumbing inspector probably won't care about the concrete as long as my pipes aren't touching it and there aren't any other foundation or slab inspections, just a county "requirement" to follow IRC 2012 or so...don't remember the exact year but it is prior to 2015.
Thanks for the input.