Poly will work fine sprayed over the magic marker. The Square already has a poly coat. DO NOT USE LAQUER-it will bunch and go bad.
I'd get an airbrush and do it gently if it were here. Take it to a refinisher to spray the poly. It will be fine. you only lightly spray the sigs.
Bootsey lives in Covington, Ky, and I see him around here occasionally and at Summer NAMM. He demos for Ampeg.
I'll spray the guitar free. Of course theres the 20 plus hours of driving...(LOL).
Be well,
R.