I think you should be able to engrave tuna meat. Below is a link to a youtube video where a laser engraves a steak and they grill it later. Not a glowforge laser though, but should be no different.
Depends what you intend to do with it later, but wouldn’t it be better to just get a piece of metal in the shape of whatever you wan’t to engrave, heat it up really high and just mark it like they do with cattle branding?