Better wash the faded black T shirt so you are appropriately attired for the unveiling of their new song as it sounds like a flanny shirt might be a bit out of place with the dark tones suggested.

Thinking through the masking dilemma and reckon that if you put one fine coat of clear on the 3-5mm edge of the maple fret board top you should then be able to mask and stain with a bit more confidence and once dark bits are fully cured reverse the masking, lightly sand off the original clear if no longer wanted and proceed to finish off front how you intended.

Or you could attempt something like this....

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If the fret board was masked the dark effect would have stayed on the back but for this build I wanted that dirty look all over. Easy enough to stop at the actual fret board level but that means you have to re-do the side fret marker dots.