An attacker could just as easily block the SSL connections - unless you're suggesting it fail if it can't check for a firmware update, but that proposal would mean that if your servers are gone the devices are bricked. Many of such devices function fine in a Lan only setting behind a nat where they're virtually untouchable.

Disallowing downgrades via a signed datestamp is about the best you can do. Anything else will either be trivially blocked or result in other user problems.