I'm writing a solo cello piece, and I'd like one of the strings to stopped at the first harmonic (specifically, the G string) for the duration of the piece; The trick is, I need this to be done without the cellists fingers, so that they are free for other strings. I was thinking maybe an earplug tucked between the string and the fretboard might provive the right amount of pressure, but I'm not sure, and I don't have a cello to test this out on. Maybe there is already an established way to do this, but I've never seen it before.
Cellists, do you know of a better way to achieve something like this?