On my first engine powered anything I intalled a cast-iron wrap-the-rope around the pulley on the flywheel of a Briggs and Stratton off a reel mower on to my bicycle Never forget one day riding around the potato fields I cam across a junk pile. There was a steel tapered table leg and I laughingly slipped it over the muffler where it fit tight. Firing it up, it made a hilarious ping ping ping exhaust noise, there was about a 1/8" drain hole for all the exhaust to exit. Of course, laughing I tried riding it and surprisingly found it had gained low end power, it would now idle up steep hills with ease and could not be stalled at low rpm, something that it never could do before. The pipe destroyed any kind of higher revs and was discarded.
There was a discussion on another forum on this subject and someone mentioned his installing Flowmasters on his late model F150 had killed low end and mid-range especially when trailer pulling. Given that almost any cam has overlap and the racier the cam, generally the more overlap hence more rumpty rump the idle. So I'm wondering IF the back pressure were increased significantly could it effectively knock off or tame down the effects of the overlap and allow the motor with overlap to make more low end power, perhaps smooth out the idle etc? ................I know your dyno is not happy at low rpm and this would really be low rpm but any chance you might try to see what a really restricted exhaust does way down low to a mild or aggressive motor at part thgrottle? ...................... That said possibly a butterfly valve could be installed to back pressure up a motor and make it perform better really low, putting around town etc.
I reiterate if this would ever work the exhaust needs to be literally strangled, scavenging is not even on the menue.... Obviously it is not for anything over 1,000-1,500 because IF it breaths beyond that then the exhaust (closed down) is not restrictive enough. If it works on idle then perhaps it can be expanded to lets say restrict flow add backpressure at 2,500rpm on a 7,000-8,000rpm motor.
