Official comment
My guess is never. This restriction does a couple of things...
1) Encourages users to purchase Pioneer DJ controllers (good for PDJ sales!)
2) Limits the number of controllers DJs are likely to use in the rekordbox ecosystem, making support easier as it's really "our own products"
So no, I don't see them ever opening this up to other hardware, just like Serato DJ allows for MIDI control with non-Serato DJ-compatible devices, but no deck/jog control.
Unfortunately, the answer appears to be no.