I have personally used uranyl nitrate mordant with and without aluminum, with the combined effect being a bit sharper. Depends on the receiver paper. But I'd have to defer to Jim's experience on that point. Regarding the use of pigment directly in the relief film and opacity, the hypothetical solution is
much finer pigments which in effect behave as if they were more transparent in relation to saturation. This can be done, but I've already got way to many irons in the fire to do the testing myself per ideal process colors. The carbon printing crowd seems completely unaware of some of the
significant advances in pigment technology which might be applicable. But nothing at this point has
been marketed for process work per se, so there are chroma issues (but nowhere near as bad as back which things like alizarin crimson and poison green were used). One more test which will have to
wait till I retire.