If you are doing this to prevent having to tear up some concrete, when a gravity flush toilet would work, it is probably false economy. Any time you can get by with gravity, which is free, performing an action verses powering a unit, you are better off. Even if you do need a pump, putting in an ejection system and using standard fixtures has benefits - more storage. If the power is off, with gravity, it's no big deal. When you have to pump things, it can get ugly quick if there is no power!