there is a frequently expressed attitude on LesWrong that as would-be perfect Bayesians, we should easily come to agreement about everything.

Hm, definitely no. I think that most lesswrongers realize that we are really really far from being prefect bayesians and having common priors and also realize how far we are from agreeing. That Helman paper surely argues with me, however I can believe that there are a bunch of lesswrongers deluded into thinking that we are already at the stage when we can easily reach agreements about everything. I am skeptical to them being more than a minority though.

Try the Aaronson paper, and the papers it references.

The Aaronson protocols address the real-world limitations of time and computing power . The mechanism through which perfect bayesians receive information is not important outside of such real-world limitations (as those are limitations for humans and as perfect bayesians exist slightly outside those limitations). The data that they currently posses and are able to obtain is.

There must be a medium. The properties of that medium determine what can be communicated and how long it takes.

I am assuming mediums where they are able to exchange whatever information they want even if they need to send it through binary - mediums such as the one here. And no, it does not matter how long it takes - the same agreement will be reached if it takes a billion years or 4 milliseconds, this is perfect bayesians we are talking about.