Not sure it satisfies your needs, but you can always save out as 16bit tiff from DPP and then edit the tiff with ACR, won't lose much in the translation.
Oh can I, thanks . I couldn't see a way to open a tiff in ACR - It only opens when I drag a Raw image into ps6, and it has no file>open to allow me to open a tiff. In fact it is a rather strange piece of software - it doesn't seem to exist on its own (not as part of production suite anyway).
This would be a work-around for sure if you could tell me how to do it??
FOr anyone else with this problem you can force ps6 to open all tiffs in ACR with a setting in preferences in RAW - automatically open all tiffs. It is still a bit of a pain as i dont really want to automatically open them all, but hey!
I normally open all files from within Bridge, in which case I can right click on the thumbnail and open in Camera Raw, or directly into Photoshop.
Added by edit - ACR is a plug-in, not a standalone program. It can be invoked/hosted from either Photoshop or Bridge, and if from Bridge can be utilized with raw files or jpeg/tiff's.