That makes sense, thanks for that. I'll likely try on some scrap wood and see how it works out before taking it to the guitar if I decide to.
Testing for compatibility on offcuts or scrap is always a good idea when using new products.

I have some of the Feast Watson Wipe On Satin Poly from the BGS, that should probably be okay for the neck yeah?
FW and Cabot's are used regularly around here. I use the Cabothane (stupid name) poly and have had good results with it.
Since you already have the FW wipe-on you should be good, but for future reference, I always recommend buying a small tin (250ml) of "regular" oil based poly and thinning it with turps to use as a wipe-on. It's way cheaper than buying the stuff labelled specifically as wipe-on poly. What you pay for is basically pre-thinned poly.

Between a 50/50 to 60/40 poly/turps mixture is a good starting point. You can adjust as desired from there. I use 60/40 with my mini gun. I've never used water based poly, but I know it's not recommended thinning beyond 10% as it's quite thin to start with.

What do you guys think of Jarrah for a fretboard?
If you're talking about just staining the fretboard, it might be difficult to get a clean line where the two meet on an all maple board and back. I've never tried it so can't say firsthand, but I would think bleed from the fretboard piece to the back carved piece might be hard to control.

That said, if you can pull it off, it might look pretty cool.