I seriously would like to have a pedalboard with, say, 5 pedals that i could map to specific functions on Emacs.

It would be terrific, particularly for Lisp development with SLIME.

Hmm.

MIDI foot controller -> USB MIDI interface -> computer -> driver -> user space scriptology ...

What do you think? That would easily give you ten pedals.

I use a 1993 vintage ART Ultrafoot X-15 I got off Craigslist --- for my guitar rig, not a text editor. :)

https://www.google.ca/search?q=ART+Ultrafoot+x-15&tbm=isch

Why not just play the piano for programming?

A MIDI foot controller isn't a musical keyboard (though such things do exist, mimicing organ pedals). It's used for MIDI program changes. I have a MIDI programmable guitar pre-amp, and so in combination with this pedal, I have access to various amp settings just by stepping on various buttons.

You got it wrong! It's more like:

C-M-handstand-q-footpedal

C-M-guru-meditate

https://en.wikipedia.org/wiki/Joyboard

I wonder why foot pedals have never caught on... Seems like they would be genuinely useful!

They have. Kinesis have offered foot pedals for more than a decade. Personally I don't sit in a way that would work for me.