The Mid and Treble settings on the Madbean project is essentially a switch between two caps, the input cap actually. Go pull up the Madbean project and you'll see what I mean in my next paragraph.
So the signal goes into the input on the left. C1 and C2 both connect to the input. If you follow through to the other side of C1 and C2 you'll see the two pads for the 'T/M' boost selector. So what that does is bring in the second Capacitor in PARALLEL to the first. Remember, when capacitors are PARALLEL to each other the ADD to each other, so if a 5nF and a 10nF capacitor are in parallel, they equal 15nF. Resistors in parallel divide.
Essentially what they're doing is acting in conjunction with the following resistors as a filter. It's like a 'low cut' or 'high pass' filter. As the signal travels through the cap and resistor to ground, you lose something of the original signal. Pedals do that all the time, but they usually find a way to either add it back in, or do it later one where the signal has been boosted and now you need something filtered out. But what's important is the value of that capacitor right? The higher the value, the farther back the 'low cut' happens. SO when you add in the 10nF cap, and raise the filter to 15nF, you add in some more of the mids. Add in a 22nF cap and you'd add in even more 'mids' and perhaps some of the low frequencies. I know there are calculators and simulators that will show you precisely what combination of caps would give you boost at what frequency, but I don't worry to much about that. I just build.
So from that, you can now begin to customize your own Rangemaster. Raising the value of those two caps will bring more mids in. I kinda want to take mine apart and switch the caps, making C1 10nF and adding in 5nF for just a bit more mids, but I usually have my rangemaster in mids mode anyway because of my amps and guitars. I'm sure if I were playing a wooly AC30 it'd be different.
So the long answer is - yes, you can tinker with this thing. I hope this sheds some light on how it all works! Maybe Brian will follow up with some math so you can make some calculations and find just what you're looking for. Links are always fun.
Jacob