Haven’t tried it myself but CDJ-3000s have been rooted.

https://github.com/connorworley/cdj3k-root

You could put a lot of time and effort into getting your own firmware for CDJs .. or you could just connect them via USB to an pi running mixxx.

What you mean? Can CDJ be hooked up to a pi using mixxxx?

The whole point is that the DJs in my collective are used to play vinal, so the pioneer CDJs just feels nice on the hand.

Yes some of the newer dj controllers r quite nice, but rekorbox just got to all the clubs first, so it's everywhere. Kinda like how Adobe is the defector for graphics.

You can use CDJs as a controller. Not with Midi, but in HID mode.

I have only tried it with traktor, but AFAIK it should also work with mixxx. Pi, laptop, whatever. And you can keep using the screens on the CDJs.

And the next user can keep using the USB, no hardware or software modification necessary. Much easier then coding a new firmware.

Mixxx, Algoriddim DJay, Traktor, Rekordox, probably Virtual DJ. Connect CDJs and mixer via USB and off you go. If you have an mixer with no build in audio interface, you would need a external sound card (a hassle because a lot of cables) or aggregate all the CDJs into one virtual card.