Probably better way to solve problem like this, bitmap CMYK shadow over Pantone background is to export PDF in higher version... 1.5 or 1.6 for example, where transparency's are supported, then to open pdf in Illustrator, convert transparency bitmap (shadow) to be only pure black with transparency, then just again export to PDF but now in 1.3 (Acrobat 4) version. Illustrator preserve pantone information under shadow in such low version of PDf cos it support bitmaps with pantones....

same decision can be used and for walls of the house, just pantone shape under gradient shape just in black with overprint..... this work well and directly from Xara...

before time, here in TG has a similar problem with one logo cant remember which exactly was.... there problem with dark (black raster) over 100 pantone was decided in the same way... overprints are extremal useful things.... and if they can be shown direct on monitor with final result as preview will be one of the best updates from Xara