I've always been curious about this. It translates into what I can use as my inlay palette on an unfinished board. Since ebony and rosewood are so popular, that's a common occurance. I've seen koa bindings on rosewood and ebony but never seen close enough photos to tell for certain if they finished and with what.
I use a fretboard oil from Stew Mac that's supposed to dry hard and provide some sort of protective finish. I have no idea if it would prevent maple from turning grey from grubby fingers. I should do a test at some point.
I used to file koa under "needs a finish". The only fretboards I'd seen of koa had finishes. Then I saw a Taylor with a koa inlay on the upper frets of a rosewood board. Maybe they're counting on the upper frets of an acoustic not getting much wear. I generally try to stick with "hard" woods. Arbitrarily above 800-900 Janka hardness. Though I've seen softer woods used.
The issue with maple is how quickly most of it turns an ugly grey when exposed to finger grease. I wouldn't mind it getting a darker, warm color and maybe some does. But I've seen some that just looked dirty and grey. More grey than the photo above. And it probably depends on the person's fingers.
I totally avoid it unfinished. I use stabilized maple if I need to use it for an inlay, but it's mostly available in small blanks. Pen blanks and knife blanks. You can get whole tops from Gallery Hardwoods though. I know a lot of bass builders do. I love the dyed stabilized material because it lets me extend my inlay palette and still use natural materials. Ok, they've been treated with unnatural things, but it's no different than the finish on the rest of the guitar and the stain on the top. It's just vacuum impregnated deep into the material.
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