sneaky message passing is about finding out-of-band channels for passing data between components.
PYK 2014-04-14: I added an object system to Functional imaging which uses $::errorInfo to bind methods to their instances. Perhaps this is a hideous thing to do, but in its first application, at least, it seems harmless enough.
PYK 2014-06-19: In do...until in Tcl, the tailcall variant needed to store the result and options from catch without polluting the caller's namespace, so used $::errorInfo and ::$erroCode for the purpose.
Braintwisters and other pages illustrate the art of rewriting procedures as an alternate way of passing information into the procedure.
AMG: The Wibble web server arranges things such that the [info coroutine] command returns the name of the channel used to communicate with the client.
PYK 2014-06-19: I had seen that in Wibble. Nifty!