The first time I run Geary after a reflash, the default keyring just gets created. (It of course asks me for the keyring password to set on the new keyring.) So maybe the hack answer is to configure and run Geary (even if only temporarily).
FWIW, the folder containing the keyrings is ~/.local/share/keyrings and if the task gets done correctly then you will have a file Default_keyring.keyring in that folder.
I remembered that some years ago I tested the app Geary on my L5 and found it useles (I dont need a GUI to write mails, I use mutt in a terminal). The app stored in the past already in the “Default keyring” the IMAP and SMTP credentials and I only have now let gpclient do the same providing the password to unlock the keyring.