Using a binary-to-octal IC, like the CD4028b, one can translate those three gate signals from the on/off buttons into eight separate output gate signals (0-7). Adding an attenuator to each of those eight gate signals lets you tune each "number" to a voltage, for example corresponding to notes of a scale.
Adding a transpose button could add a (user defined) constant voltage to those gates as well, letting you play 16 different notes with only four buttons. Since space is always an issue in modular synths, this is pretty neat in my opinion.
Because the whole thing is based on gate signals, it's also easy to OR-wire the buttons to individual gate inputs, to play the buttons with external gate signals form other modules or button controllers.
Here's a mockup how this could work in a 2U AE Modular module:
The trimmers would tune the eight notes and the transpose offset voltage and could be accessed through the front panel. I guess one could also just put them at the back of the module and add bigger buttons on the front panel instead.
I also added a CV input for an external analog transpose voltage (e.g. form another CV sequencer).