Its always going to be an issue, but on the basis some exposure info is better than none I always set my HDR images up with the data for one of the exposures.
I don't have consistent policy on which one, but would using "common sense" normally choose the one which is right for the "main part of the image", ie the principle land exposure rather than the sky/water/shadows if it is a landscape.
Other than that you have to just add the rest of the info in additional notes. I often put something in the EXIF comment field but AMASS doesn't grab that, might be cool if it did.