To collate another option, Tiffen glass was posted about last week:
Official JVC RS4500/Z1 Owner's Thread
Thanks for the hint, I was lucky enough to find a Tiffen 100mm clear filter on eBay for a very low price a week or two ago. I've since built a projector port around it and the results are promising, I ended up with an 80mm opening after making a cardboard template, there's a small gap either side of the image so a small margin for error and adjustment.
I made the opening a lot bigger vertically than it needed to be by mistake, I got the plug upside down while marking it out!
Even with Tiffen glass there's a visible colour cast, I'm surprised considering Tiffen's reputation. I re-ran JVC autocal last night and that has corrected it out without any problems.
What I didn't expect was an improvement to blacks and therefore contrast, I guess there's sufficient stray light from the projector for the port window to make a difference. It's a bit like an iris in the light path I guess, I tested with the port plug before I fitted the glass and the improvement was there, immediately noticeable. While I was doing a final opening clearance check it was very easy to fit and remove the plug.
Plug
Rear view, from projector shelf.
Front view with projector re-installed.
I'm using almost all the vertical lens shift and almost the widest zoom, hence the view of the bottom of the lens cover.