Thanks folks.
I'm going to do a couple of test pieces as I just need to get it done, and I don't want to be redoing the big piece if something doesn't work.
I'm going to try just lacquering the bare wood first as quite often that will darken it in the same way that my white spirit test did. Some woods darken a lot, others barely at all, I'll see what happens.
If that doesn't have a satisfactory result I try a sepia calligraphy ink and see how that goes.
@Andyjr1515, thanks (as always) for the advice. If I were to slurry and buff, do you think the differing shades of wood would be an issue? Would the slurry of darker wood and Tru-oil stain the lighter areas?
I'd only be doing the slurry to grain fill as the finish will be 2k lacquer whatever happens. I know the lacquer will work well with wood and be completely waterproof, so no risk in the bathroom.
I don't know if you saw but this was calligraphy ink then lacquer on a Kay that I did, this finish is what I'm aiming for (not the colour though).