Sam,
It is easier than that. Just name the folder ".PMIngestIgnore" using Terminal.
Let's say you have a volume named "Archive" that you don't want to show up in the "Ingest Disks" list. Open Terminal and type this command:
mkdir /Volumes/Archive/.PMIngestIgnore
That's it. The volume "Archive" will no longer show up in the "Ingest Disks" list.
Scott