I get a variety of untagged jpegs & tiffs that I want to edit in ACR that aren't necessarily sRGB. Why can't ACR mimic Photoshop's "Ask" dialog to display the image in another working color space besides sRGB?
It would make sense to have ACR ask for a missing profile if you are working on one image at a time, perhaps via drag-and-drop or File / Open in Photoshop, itself, but there is an issue when using ACR with Bridge to open many images at a time, which is perfectly normal in some peoples' workflows.
So if ACR does prompt or assume and convert on the way in, it should be a setting that can be turned off at least for multiple image opens, and there should be a way to batch-fix missing profiles in Bridge, perhaps by filtering on missing profiles, and allowing Bridge to update a profile NAME into an image's metadata which ACR could use for an implicit conversion. Judging what a missing profile should be could be done better when the images can actually be seen rather than in warning box as part of ACR.
If the profile named in the image's metadata didn't exist on the current system, it probably should generate an error or at least that option should be configurable.
What process is it that leads to your getting untagged non-sRGB images to work on in ACR?