It's possible to cast a vote via Keychain without logging in, at dPoll.
It was not working for a while due to a structural change in Keychain extension, but it's fixed now.
In any poll detail, just click the "Vote with Keychain" link.
You will be asked your username:
After that, you will be requested to vote (post in Hive blockchain's context):
I know the integration is not best considering there is no log-in mechanism with Keychain, yet. But, it's at least possible. :)