how much of a propulsion system could you feasibly pack in a cubesat?

Very very little, but not nothing. I've seen a few tiny deployable solar sails and a few tiny electric motors in my brief research for my video. They seem mostly experimental, to test out theories and miniaturization.

Enough to maintain your CubeSat's orbit and do a bit of pointing, but you'd need something bigger to have the propellant and power to get out of the solar system