Here's where we are so far...
Our best guess is that the cat is in the raised section to the left, i.e. close to the pipe from the exhaust manifold, with the silencer gubbins in the bit to the right. So one plan of action is to cut into the raised bit and have an experimental dig.
View of the bottom of the exhaust box.
And... for anyone who's interested, this is boniyam's Tenere still sitting in Bikerhome in Ouarzazate waiting for parts.
