I guess I'm puzzled why your clients would object to face mounted prints, and yet not object to a super high gloss FujiFlex print. The print has to be protected somehow.
My preferred print is Epson Legacy Baryta mounted to diBond and then glazed with a cloth liner separating the print from a piece of anti-reflective TruLife acrylic, or even TruVue Museum Glass or a piece of Groglass ArtGlass. The anti reflective property allows the surface of the print to dominate and you get crisp vibrant colors with great detail.
As I said before, no I do not use the Epson Proofing White Semimatte. I use Epson Premium Semi-Matte. So far it's the best bond I've been able to get and allow's me to get a look very much like the fujiflex when face mounted, but because it's high quality ink jet I have more gamut, smoother transitions, more accurate colors, and better detail, and inkjet has proven superior to chemical photographs in every longevity test out there.