Here's my thoughts:
In theory, an aftermarket exhaust system (whether it be slip-on or full), is a straight pipe with packing in the silencer. I mean, look at the silencer for a sparks pipe, it's just a big hole all the way through. Granted they're all tuned slightly different, but the concept is the same.
I did this with my Z400 and it worked beautifully. I cut the silencer on both ends, then took all the guts out, got some wire mesh (get some strong mesh). Take the wire mesh and form a cylinder out of it. Then buy some 4-stroke packing from your local dealer and wrap it around the mesh. Shove the packing and cylinder into the shortened silencer.
I can't guarantee the results you'll have with a YFZ, but for the Z400 it gave me a ton of low-end torque. It gave me atleast as much as I have with my full Sparks pipe now. You will most likely need to re-jet (as I did). Be prepared to give up a little bit of top-end though. As I see it, it was worth it, because I don't ever see 5th gear on an MX track.
Also: If you do this, make sure you have a way to repack your exhaust, I made this mistake when I made mine. I also used thin wire for the mesh, and it crumbled up and all the packing came out. When I needed to repack, I couldn't do it effectively, so I bought an exhaust. I'll be fixing the stock modded one when I get a chance though.