not sure, however...it handles 2 just fine and RB uses upto 4 inputs. I think it would be very possible.
Can Rocksmith process a keyboard output ? If the program knows it is a keyboard, easily.
In fact it would be easier to translate than a guitar signal. Keyboard notes are straight forward and the game can easily tell what keys
since the new xbox is on the horizon, I would not be surprised to see some of this stuff show up in a rocksmith 2 on next gen systems. they should have more than enough processing power to handle more/different instruments.
I can think of enough features I'd like that are just focused on guitar/bass, that I would be entirely happy with a Rocksmith 2 that did not include stuff I can already do on Rock Band (drums/vox/keys).
And us drummers with elctronic kits who could be playing too are left out in the cold.... and with your message i give a middle finger up!