I think the discussion is very interesting, so I'm eager to see how it develops.
Before building this contraption, I've also investigated virtual batteries, then eventually decided it would have been much simpler to use real batteries rather than trying to simulate their effects. This is, however, something that I'd like to research further in the future.
Well, the crux here is that real batteries die eventually. Also, I hate batteries. As a parent, my hate for batteries comes from the sheer amount I've used in the first 8 years of parenthood and also by my lack of understanding, whether on purpose or not, of how waste batteries are handled.
If a battery simulator can be made to ape all the good things about batteries in these fuzz circuits, without waste, and frankly with more tweakability, then I'm game.
The advantage of a tweakable battery simulator is as follows:
1.) Control over voltage;
2.) Control over simulated battery impedance;
3.) Control over simulated battery capacitance
With a battery, your stuck with the electrochemical properties of the battery itself. By modelling these things as independent parameters, you can tweak the sound to be perfect for each device.
My rough idea in my head is for a single 9v input feeding 4 separate battery sim circuits feeding up to 6 devices. An ON-ON-ON DPDT could switch in 1 Diode, 2 Diodes in series, or no diodes for voltage drop, A potentiometer used for series resistance, and an ON-ON-ON switch for some different capacitance values.
This way, you could essentially set it as a fresh battery, slightly depleted, and almost dead and do so for each device.