Recently installed art tiles in a cancer survivor memorial park already ruined by crackhead scribbles.
Polycarbonate sheets in front. But yeah. Definitely have to be careful with alcohol. Thankfully it isn't a heavily detailed patch. Could do a test patch on the blue with relative safety.
Though, if the paint/glaze is done properly for exposure to the elements it "should" be okay. And if it isn't, sharpie and alcohol is the least of it's worries I think.