I like the bobbing displacer idea best. A negative buoyancy mass located in a corner or at the side of the build tank could be supported by a linkage to an eccentric on a small motor shaft. The eccentric offset would be very small, and the motor would make one revolution at the end of each print layer, then be still during the next print layer. Is there access to the control signals on the peachy board which signal the end of a print layer, or is there a flag in the software? Keeping the rotation speed slow enough to prevent waves might be a challenge, if using a simple dc motor (like the vibrator motor from a cell phone). Maybe it could be controlled with a digital circuit using PWM (pulse width modulation). If the weight were sufficient to return the eccentric to the bottom when power is off, no feedback for motor position would be required. The motor controller would drive it to approximately 270 degrees, and gravity would complete the rev. DDS