good thinking.. i never was quite sure if its a general cleanout or a drain or was meant specifically for cleaning that little 1/2 inch tube.. I see that bolt is almost perfectly inline with that tiny tube..
If you stick a pencil in the bolt hole, one end of the 3 inch long tube is about 2 inched away. So, a 6 inch long pencil would be able to go all the way straight through that tube.
The tube is welded to a baffle, the only baffle in there. It is welded sideways, across the muffler, and seperates it into two halves.
Whatever you push out of the tube will end up behind the baffle and will eventually be blown back out the small tube. The main exhaust tube circles around and through the baffle and is probably too large to get totally clogged up, but that small one is very likely to be restricted by carbon.