That's exactly what i'm doing right now. I don't have anything like a sump or fuge so i'm modding my AC70 to hold sand, LR
rubble, and macro. I know that people also do this with their AC110's since they're alot bigger and can hold more things inside like a 6 inch heater, retardedly small pump for flow, etc.. I think it's a great idea for people on a budget, or that just don't have the space for another tank.
However, the only problem i foresee is the macro eventually jamming the impeller in the pump... But maybe by then you could just get another Filter to replace the one that broke. Not sure, but i'm still doing this anyways.