Apart from closing TB or telling your firewall to keep TB from connecting to the net (this will make TB create error entries in its log and start beeping) that can't be done.
Another option would be to disable all automatic checking and only check your mail manually or even create a schedule in the scheduler that makes TB check your mail at 8:00, 10:00, 12:00, 14:00 and 16:00, no matter how long it has been running.
I've seen the offline wish before so it's possible that it's somewhere on the wishlist, though I wouldn't know anything about its priority.
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I'm just a user of The Bat! I don't work for Ritlabs.