Is there a linear DC ESC out there?

[Wombii]

Very nice work on the crane. I don't know any linear ESCs, but I would test using an ESC with a different PWM output frequency. A higher frequency might better emulate a linear voltage.

Have you considered testing the small ESC board inside a standard RC servo? They're designed for small DC motors with a high gear ratio and having equal performance in both directions. I would guess there's a higher chance of finding a high pwm frequency output on a board from a mini or micro servo. Open the servo, carefully pull out the insides and center the feedback potentiometer.