Sam, I've noticed you haven't got a response yet.
This may be due to the specifics of what you've posted. The first part What do you reckon to an Orion with California special electronics, maccassar ebony top, purple heart fingerboard and a custom F hole bound with silver? seems to indicate you already have a clear idea of what you want. The second part I need a really nice dark peice of maccassar to contrast wiht tthe silver. failing that, does anyone have any suggestions on top woods??? is a VERY general question.
But I'll bite. As far as the 1st part goes, I'm a neck-thru man, and a neck-thru advocate; I think they're superior instruments, and so, I'd advocate a Spectrum over an Orion. I'd also recommend the getting the Spectrum electronics, and upgrading to a 3-position Q switch. The Q is, IMO, an essential aspect of the Alembic sound, and for that reason, I'd consider a Spectrum with Spectrum electronics to be preferable. As far as the other details, it's all personal choice. Ebony is one of my favorite woods for both appearance and sound -although I'd never get it for a top, as my preference would be to put the ebony in the neck as a laminate or two -where it has MUCH more impact on the sound. Especially if you're set on NOT having the fingerboard be ebony. And having an outline of an F hole in silver doesn't grab me, but it sounds cool, and if you like it, go for it. I doubt they'd do an actual (hollow) F hole... I don't think Alembic does true hollow-bodies.
As far as the second question, I think it's too broad for anyone to meaningfully answer. Alembic stocks at least a dozen different standard and upgrade woods... if you poke around on the site you can find the standard woods and other options fairly easily.
But what would look the best to you? Only you could say. I guess if Ebony grabs you in particular, you might consider/take a look at dark Coco Bolo and/or Rosewood (both are famous for their sound -like other Rosewoods), and possibly Ziricote or some dark Walnut.
A Purpleheart or Vermillion top would likely contrast a silver inlay as well.
If you dig around and use the search for Spider (or maybe it's Spyder?) basses, you can see a couple Alembic Exploiters with silver web inlays (ala John Entwistle) on Maple tops -to see how that grabs you appearance-wise.