I just wanted to mention that a rusty version of this is basically what is inside the stock MBK Model 7 exhaust: http://www.1977mopeds.com/motobecane-vlx-header.html
I was thinking that making the header have enough back pressure could be as easy as covering the end of the header with sheet metal and then making a 10mm hole in that.
Basically the header with a can with a small hole that looks like this: http://www.mopedarmy.com/mediawiki/images/3/34/Step_2.JPG
Any ideas if that will work? Is a straight header just too open? The pic has an expansion chamber. My proposed setup would just be a straight pipe with a can on the back.