Here ANYTHING with a sewer pipe REQUIRES a frost footing for long term survival of the connections on the Sewer piping....
How are you insulating the Banjo? It won't improve your rep. if they have blow all the water lines during each sub freezing moment...
Use of brick dura-wall in every course would reduce the numbers of pieces the crapper falls apart into, but would actually increase the demo costs when the structure was rebuilt to code with the funds the home owner gets from suing an obviously incompetent builder...
One could use 8" bed 4"high "buffalo brick" that could be grouted & rebarred every 4' in ALL directions or regular rock face CMUs....
Please post the H owners response the pouring of an useless chuck of Concrete...
Cheapest quality fix, is to trench around the 8 x 8 SOG and make the kybo 9.33' x 9.33' out side to outside wall dimension.....room for a shower and a pissoir.
Even climes with out frost need structural support and Rat barriers....I suggest a serious number of hours a week learning your trade before you "kill" anymore patients with your profound ignorance. A "rabbi" or mentor could save you hundreds of thousands in losses from Omissions and Errors, if consulted before acting....