If command-control is not an option, you could install engine electronics that has built-in speed control. Not plug-and-play and not low-cost but a MTH PS2/PS3 upgrade kit could be installed. I'd expect the conventional speed control in PS2/PS3 going downhill (and/or uphill) would be as good as the 95 rheostat or the diode method.
I can't imagine anyone doing this since the rheostat or diode methods are just a few bucks.
There was a short window of time when command control was just taking hold where it might have made sense for an electronic reversing-unit manufacturer to offer speed control similar to PS2/3 conventional speed control. But that window is closed.