Have you tried asking in the Bridge forum?
This is weird. I am able to add metadata to ORFs from my E-P1 if they were
shot this month (August), creating XMP sidecar files for each. However, when
I select an ORF from July, and select "Append Metadata" from the dropdown menu
in the metadata panel in Bridge, everything is grayed out. If I select all of
my images (July and August ones) and try to append metadata, I get a message
to the effect that some images cannot be modified.
No. It appears to be a ACR problem, not a Bridge problem. Investigating further, I discovered that I cannot open the July ORFs in Camera Raw, whereas i can open the August ones. for some reason, ACR doesn't think an E-P1 ORF from July is a RAW file. Other than the date, there should be no difference between the files -- same camera, same 17 mm lens.
You were right. Investigating yet further, I finally figured out that it was a Bridge problem after all. Clearing the Bridge cache for the July ORFs enabled me to add metadata and open them in ACR. I'm guessing I must have inadvertently pointed Bridge towards the July ORFs a couple of weeks ago, creating a cache that messed up ACR 5.5. Thanks for taking the time to try to help me.
No problem. Glad you were able to clear it up.