breaking out the TWI pins would be a very good idea for direct integration with aleph, modular, and New Things. just two holes for 0.1" header pins would be fine.
(as you already know i’m sure, TWI0 pins on teensy 3.1/3.2 are (18, 19) by default, (16, 17) alternately. (37, 38) for TWI1)
adding i2c slave transmission is about as simple as it gets:
some day we will want to have a proper discussion about a unified message format for this kind of thing (i2c controllers)
(i’d be interested in one of these if i didn’t already have Too Many Faders And Teensys around. though i have to say the PCB quote seems a bit expensive. hope you get enough interest for a good break) actually on second thought put me down for a PCB and batch of alps, if it helps.