I've set up an online watch-only Electrum (2.6.4) with the xpub from my offline Electrum+KeepKey walet, created a transaction and loaded it into my offline Electrum to try to sign it with the KeepKey.

The problem is my offline Electrum tries to connect to server instead of instructing the KeepKey to sign the transaction.  It gives me a "Server did not answer" message and fails.

I tried this with a Trezor and it works fine, but not KeepKey for some reason.

Edit:

I was wrong, offline signing does not work even with the Trezor, either.

I thought it worked but the test was not fair because i simply unplugged the internet from a synced wallet to test it.  If the offline Electrum is not synced when trying to sign a loaded transaction, the Electrum keepkey/trezor plugin tries to connect to the server instead of simply instructing the hardware wallet to sign the loaded transaction.