I posted a link to this question to the Bike Friday Yak list - tandem riding is popular there - and got back a few responses, this one in particular. I'm posting this here simply because nobody else seems to have an answer; If this isn't typical, please feel free to edit this answer.
A standard signalling vocabulary seems to not exist. Teams need to ride together a lot until signals evolve in time, and every team seems to develop their own shorthand organically. This may not be the most efficient way to do this, as the loss of one rider in a team essentially means that the rider left will need to form new signals from scratch.
From poster John S. Allen:
In my experience, it's either "slow down, I'm scared" or "wow, this is fun!"
But seriously, see this by Bill McCready:
This article spends a lot of time on how the team needs to function as a unit, getting a rhythm going. McCready uses the example of mounting the bike and starting, but it's easy to see how this applies to shifting, leaning into turns, etc.
Also these pages by Sheldon Brown:
In particular, this quote is relevant to this question:
The team becomes more than the sum of its parts. An experienced tandem team develops a very special level of non-verbal communication, via subtle weight shifts, variations in pedal force, and general empathy.
After a few hundred miles together, you will find yourself coasting at the same time, shifting without the need for discussion, and and maneuvering smoothly even at slow speeds.
This is not just a matter of each rider's acquiring captaining/stoking skills; when two equally experienced teams switch stokers, something is lost, and this special communication doesn't happen...it really is unique to each couple.
While I can imagine that things might be different in competition, it seems there are no standardized communication cues. A tandem team needs to suffer through the initial stages of learning to read both the situation and each other.
(Again, I stress that this answer is only the results of my research. If the information here is incorrect, please downvote and leave a comment.)