I´ve been trying to do exactly what OP is refering to.

I´ve started Electrum with the -o option and successfully restored from a previously created seed.
When I try to sign an unsigned transaction created on my online computer, I get an Exception "Adress not found" showing the public key adress which is the input for that transaction.

That public key adress isn´t showing up on the offline computer but I would assume that it could be derived from one of the private keys!?

What am I missing?

edit:

nevermind, found it:

executing "wallet.accounts[0].create_new_address(0)" in the console did the trick... described here https://bitcointalk.org/index.php?topic=344115.0