I don't think Quark can pass tagged RGB through to pdf.
Quark is aware of ICC profiles attached to placed images and can use the ICC to change to a different color space but postscript doesn't support ICC so instead quark converts the ICC and image numbers to CIEbased color using Based on Postscript color management, most RIPs see these objects in a PDF as a whole different type of colour and treat them independantly so although the results can be almost identical its a different process that gets it there.
If you followed all that above be aware that to even do this you have to Create a new Output Color setup, create a new one based on As Is and switch on include ICC .
To be honest you are much better off letting the conversion from RGB to CMYK happen earlier than the RIP, either do it in Photoshop or get Quark to convert to CMYK using a custom set-up (the only major downside is that by using Quarks CMYK output space instead of As Is all the PANTONE Colors get converted based on Lab>CMYK rather than using the CMYK numbers). Remember the PDFX is designed to give printers something they'll find hard to go wrong and RGB is a bit of a stretch.
... there was the puzzle of why the sun came out during the day, instead of at night when the light would come in useful.
Terry Pratchet