Sorry about calling you Jon
For some reason I thought infrit posted this.
I don't have any experience, none whatsoeverl with bows having glass belly and no backing. My hunch would be that being fiberglass returns to its normal state that the bow wouldn't take any set but I don't know that for certain. Maybe someone else will chime in.
I unstring all of my selfbows including bamboo back and belly bows. You probably gave the pawn shop owner good advice none the less.