Thanks for the information. I guess I need to spend more time learning Electrum.

After digging into it, it seems Iwas wrong, the Trezor+Electrum 2.6.4 currently does not work for truly offline signing. 

When loading a transaction from file/text/whatever to sign with the Trezor, the Electrum plugin checks to see if the necessary inputs are present. In a truly offline Electrum wallet, synchronization will never take place, so the plugin will try to talk to the server and fail instead of simply instructing the hardware wallet to sign the transaction and let the online Electrum reconcile whether or not there's money to spend.

BitcoinNewsMagazine, I understand this is not a standard use case for the Trezor, but if I can get it to work, it will certainly give me an extra bit of security.  Peace of mind that my (self-generated) mnemonic will never touch an online computer means I don't have to worry about the unlikely chance of the Trezor getting compromised (however minuscule the chances are)