Thanks tbirdvet,
I've made some progress and here is what I have found. I have been using the procedure you describe to identify the desired mailbox in the TM back up, move it to a staging area and import it into Mail from there.
What I'm finding is that that box doesn't show in Mail - the "import" mailbox doesn't get created even though the import appears to run to completion. I have an idea of what the problem might be.
Is it possible that this mailbox I'm trying to bring in is actually empty? See the following:

This is the backup mail folder I've retrieved from Time Machine. I'm trying to import Cube.mbox. The backup that I've retrieved shows an entire hierarchy of mail.
I can point start the import to the top of the hierarchy and then select only Cube.mbox or I can move Cube.mbox to another location and import it from there. Both ways appear to complete with Mail indicating to look for the new Import mailbox. But it never gets created.
If I point to the top of the hierarchy and select everything to back up, I do get the "Import" mailbox created but the hierarchy underneath it looks like this:

The import only seems to have expanded the hierarchy under the first item in the list [Gmail]. And even there, not all the boxes are filled out. It never works its way further down the list.
Also - when you look at the size of the folders in the backup, most of them are too small to contain the mail I think should be there. It makes me think that it only contains pointers into some other cache of stuff. Anyway I'm confused. Clearly the TM backup doesn't contain what I think it does or I can't get it reconstructed by the mail import process.
Thanks,
Allan