One option is to forget about going below the slab and go above it. You can get a macerating toilet that pumps through a small diameter pipe to connect with the existing drain in a more convenient place.
Here is a link to one such system:
Note that I have never used one but my neighbor has one and it seems to work ok. Their pipe runs up the wall, through the ceiling and then back down into the kitchen. Make sure to glue the PVC!!!