Are there existing extensions to behavior trees that fascilitate node communication?
The most mathematical paper I have seen on Behavior Trees is How Behavior Trees Modularize Hybrid Control Systems and Generalize Sequential Behavior Compositions, the Subsumption Architecture and Decision Trees. It is the only paper I have seen that defines Behavior Trees as a tuple e.g.


  A BT is a three-tuple $T_i = \{f_i, r_i, \Delta t\}$


Since Behavior Trees are related to Finite State Machines (FSMs), they mention Petri Nets which are also related to FSMs. They're a message passing / concurrency system, but weren't mentioned really in relation to message passing on Behavior Trees. Maybe it would provide some inspiration since they are highly developed message passing formalisms.

Other than that, there may be some private work done on message passing in complex Behavior Trees such as in game companies, but there doesn't seem to be any public resources on it. Maybe it is an artifact of the way behavior trees are developed, i.e. it seems to be done through node-based graphic interfaces, so less source-code-centric:



Probably best to just come up with your own system it seems. Maybe even try asking at the gamedev.se, they might have some insight. Hope that helps.