On second thought, "Notes" -> "Reference #" is a bad idea. :

For now, I'm moving data from "Label" to "Reference #" with the Tools/Advanced menu until that glorious day when any FLAC tag (preferably "Catalog #") is mapped to "Reference #" in OCD. 8) (I know this is WAY down the list of priorities.)

Just a thought:

While testing, I found that when "Composer" is chosen in the Tools/Advanced/Fields/Source
dropdown menu, "Reference #" shows up as a possible destination.

1. Doesn't make much sense for "Reference #" to be a
destination in this instance.

2. It doesn't work, anyway. No data is copied/moved.