Pretty much anything can be done.
Look to the under engine routing of the Free ride 350, going into 2 mufflers.
I've not a thorough idea of available clearance under a Sloper 'Berg, but there may be room to make a resonator / Helmholtz chamber (it doesn't have to be 'round' shaped) under the engine, to assist with 'tuned length' requirements. Perhaps there's room for a bifurcating header, through / under suitable gaps. Then, re-merging into one secondary, before the std (location, at least) muffler. Ovalized / 'Cobra head' shaped pipes, 'D' shaped pipes, there are many options in header shape, other than circular, that work well.
The suggestion about the pipe going through the swingarm, brings a smile to my face. I grew up, surrounded by motorcycles. Amongst the many types, my Father and Uncle, kept 'tilting at windmills,' with CCMs. A few of those, ran a pipe, that the end of, went Down through the swingarm, the exit pointing straight to the ground .With either a laughingly minimalist 'muffler', or none at all. Pointing to the ground, it 'sort of' kept sound in check to just ear splitting levels, but when you went over a jump, it was like a WW2 bombardment.
Probably even better, was the dust storm, emanating from underneath the bikes, especially at the start. In dry , dusty OZ events, it was a classic sight, that freaked out a lot of blokes, who had No Idea of what was going on. That, combined with the roost from the, at times , 580 /630cc engines, could make it bloody hard to pass the bikes. In the wet / through water crossings, it was just as hilarious, with Huge steam clouds behind the bikes - remember, people weren't used to water cooled bikes overheating, in those years.They basically, didn't exist.