I think you’re going to have to open a ticket to get a new hook added into groups_delete_groupmeta so you can process the meta info before it gets wiped out.
I figured out a work around.
The whole group delete process is started by hooking the ‘wp’ action at priority 4. I created a function at priority 3 to save the information from the groupmeta into a global variable and use that later on. Nasty, but effective.
It’s probably still a good idea to open a ticket and request a new hook.