Not that I think this is a good idea, but my brother had a recommendation to bronze an article of clothing that he had signed by some musical group.
It was in his "hippie" days and I think it was a Phish hat, so maybe that explains a bit of it.
If you decide to clear coat it, make sure you try that on something else first. I would wonder if the clear coat solvents might ruin the sharpie ink.