What Label material resists steel abrasion from guitar strings?

A clear plastic printed on the reverse... polycarbonate maybe?

Polycarb would be my suggestion as well. 3M has an extensive line of silk-screen-able, self-adhesive PC labels.

What text will you be putting?

various image and/or text will be imprinted. Why would it matter?

I've played guitar for many years and have worn many rosewood fret boards down! My Guild is in a bad state at the moment with some pretty deep worn patches! I think the placement of the sticker is important too! Where are you going to position them? I don't think any sticker would last too long within the first 16 frets whatever the plastic!

Find a local label Mfg. and ask for a 3 mil thick polyester, or at least 2 mil thick. The label should be printed and next a clear adhesive transfer tape added to the printed side with a release liner and die cut to your shape. Your final label will have the printing protected with a top quality clear polyester film absorbing the abrasion.

And while you're at it, ask for the frets to be made taller!

Isn't the rosewood treated with some kind of oil to preserve it and prevent it from drying and cracking? I would think you'd be hard-pressed (sorry) to find a sticker that would reliably adhere to an oiled wood. If your market is in new guitars, you could make a solid plastic fretboard with plastic inlaid designs so that the color runs the depth of the fretboard. That way as the plastic wears (and it will) the design will remain.

Hello interjecting here just a bit.

When we are manufacturing a decal here the we want to hold its appearance we do what has been suggested previously. We use a durable material such as a poly carb or polyester. We print on the back so the print shows through the material and then apply an adhesive to the material over the ink to adhere it to whatever so the print shows nicely.

The materials will all wear about the same when using a clear as scratching will occur on all of them. Even the best of poly carbs.

As far as adhesives go there are many that will stick in the most adverse environments. However I would suggest that if you want them replaceable then you might want to consider a less aggressive adhesive and one that is promoted as a removable and not permanent.