Depends on the engine and installation. I've seen plenty of setups that would flood quite nicely if you pump fuel into the muffler, I even have a handful of planes that do it. If I forget to pull the line from the muffler, the engine floods FAST when the tank gets filled.

On cowled installs where you can't reach the carb or muffler fittings, I usually mount the engine on it's side, and that totally prevents flooding from the muffler. Inverted 4-strokes are also not a problem, and most inverted 2-strokes have the muffler outlet lower than the engine's exhaust port, but not always.

On upright engines, it depends a lot on the shape of the muffler (and sometimes the landing gear of the plane or slope of the pits if any). Some old engines have the exhaust port on the bottom of the muffler, while on others the engine's exhaust port enters the side of the can, giving plenty of volume to catch fuel before it drains back into the engine.

Btw, if you raise the piston by turning the engine up to compression, you block the exhaust port, and that will help keep fuel from draining back into the engine.

I love that idea of using a T in the pressure line and plugging the muffler for fueling, I'll have to set that up on some of my cowled engines where I currently just let some fuel come out of the muffler. I already usually use a 3-line tank because I find using a fuel dot is the simplest way. Reaching the carb line is just not possible with a 4-stroke hidden inside a cowl.

Oh, I forgot, on my cowled engines, I can usually find an angle that allows me to see back into the cowl and look at part of the vent line that goes to the muffler, even if I can't reach it. Sometimes I can only see a short bit of a loop and not the end. While I'm fueling, I watch that line, and when I see fuel, I stop the pump, and that reduces the amount of fuel that gets into the muffler.