I found a way to make filt with no wavelength, by setting the ctype to the maximum value.
Unfortunately, it's still blue, and acts the same as normal blue filt.
Yes, black FILT has mostly been prevented from doing anything strange, but there are still a few places (like the wavelength drawing in the HUD) that don't yet check that there is at least one valid wavelength present.
However, try FILT with black deco. (Photons cannot be decorated though)
You can still get BRAY with 'no color' with some filt logic.