Of course, it will accommodate a pipe tunnel. It has all the room you need to make a pipe tunnel... by my drawings even the Eather #8 will fit INSIDE the fuselage with just an opening in the back.
If you look, the holes are there for a pipe in the pass through area.
In fact, the T-Rex has more pipe tunnel room than an Impact (due to the fat airfoil of the Impact).
I think the T-Rex will be one of the most accommodating designs out there, now or in the future. It will easily accommodate a piped two stroke, a rear exhaust muffled two stroke, side exhaust 2 stroke, 4 stroke, inverted mount, side mount, or even right side up mount if that is what you want.
It will also accommodate side exhaust engines with wrap around header mufflers in the inverted position. Like if you want to run a ST 51/ST 60/ PA side exhaust with a wrap around header muffler to make the exhaust come out of the rear.
It will require a long extension muffler for side exhaust, or you could go the other way and use a short extension muffler and keep the muffler inside the cowl.