Moving individual Mailbox to external drive

Does anyone know if it's possible to move a "Mailbox" in Apple Mail to an external drive? I have a relatively small SSD drive, and I'd like to create an Archive mailbox on an external drive.

I don't want to move the entire ~/Library/Mail folder - just the Archive mbox.

If I create a MailBox called MyArchive, I can see the .mbox structure in ~/Library/Mail/V2/Mailboxes/MyArchive.mbox.

Is what I'm trying to achieve do-able? I know there's "proper" mail archiving utilities, but if I could just store the native emails on an external disk, it'd be good enough for me ...

EDIT: As discussed below, I've tried moving this folder to the external drive and creating a symbolic link to it, but with this in place Mail.app doesn't see the mail content in the folder - I can see the list of mail items in the mailbox, but opening any mail in the folder results in a "blank item". I also see the error

kernel[0]: Sandbox: Mail(99229) deny file-read-data /Volumes/External HD/MyArchive.mbox/ (followed by the path to the .emlx)

in Console.app.

Is there any way to "override" Mail.app's sandbox settings to allow it to read mail from the external path?

Q: Do you want this archive to show up in mail, or just be available should you need it?

If you just want to store the messages "offline" but have them around if needed at a later date you can select the mailbox in Mail, then select Mailbox -> Export Mailbox... (make sure you select "Export all subfolders") and save the Archive. Then you can delete the mailbox from mail and pulpit back in when needed.