Flooring – Faking a hardwood floor or flat surface in a carpeted office

Without the ability to dry on both sides, your hardwood flooring will eventually warp and crack unless you completely seal the surface.

You can use woods like Ipe and Teak in wet situations (and indeed, those are the woods traditionally used for ship decks) but I don't think Tasmanian Blackwood has the same resistance. There's at least one amazing bathroom designer here in Houston that built an Ipe shower floor...

... but that's got the drain below a 'tray' made out of ipe that can be removed so that it can be cleaned, refinished and/or repaired easily. It's strips of Ipe with gaps between them that are then nailed to cross-pieces that sit in the sloped pan of the shower. I'm not totally clear from your posting, but from what I think you're looking for something like that for your entire bathroom?

The only other alternative that I can think of for you is face-nailing the wood, and then sealing it completely with an epoxy polyurethane. However, this floor will be extremely slippery when wet.

But, in general, wood floors in bathrooms are a bad idea and are something that you should rethink.

In two old homes now I've shredded the wood floor under my chair, monster splinters eventually emerging. I work at home in semi-rural New Hampshire. I think they're very old pine floors, so softwood. I plan to try Shepherd Brand Urethane Casters after putty and repainting.

From the manufacturer's site, "Nylon tread for carpeting, and urethane tread for hard floors." Much buzz on Amazon about these.

Edit: After months with the new casters there are no new paint flakes or splinters. I haven't repainted the floor yet, but it appears these casters will do the trick.