I have been using Baryta paper (and advocating its use) for coating any gelatin based material and some non-gelatin based materials for several years now. I have coated cyanotypes and others, but Pt/Pd does not work as noted above. Unless - you put on a sizing layer to isolate the gelatin.
Baryta is made up of gelatin mixed with Barium Sulfate coated on DW photograde paper. I have samples of these products and am very very happy with them. I have posted prints in the Emulsion forum.
As for costs? Contact the Formulary. I have no idea what these may cost.