Here's a find from my friend Fred: Offline messaging exists, but in a hidden way.
Before you try this, you and your friend both should have chat archiving enabled, especially for that user. I dont know if it works with 'Off the record', it should'nt technically.
As your friend to logoff, try sending him a message now.
You will get an error message like:
blahblah@gmail.com is offline and can't receive messages right now.
Now ask you friend to check his chat folder, he should find that as new entry.