I think I may have resolved the issue - it's possible to set a colour profile in Camera Raw by clicking on the profile information at the bottom of the window (my Camera Raw was set as Adobe 1998; I've now changed it to sRGB IEC6) - by matching this with the PS colour profile it seems to export everything correctly. From a couple of test uploads the colours of my uploads now match the ones I see in Photoshop.
There's the right way, the wrong way and the railway.