Welcome to the world of abusing the shit out of Ant. My first full time job was developing Ant in unholy ways. Tens of thousands of lines of Ant at least, doing significant logic. If-then-else, for loops, math, procedures, date-time math. I stuck it out for a year. It was a year too long.
I like it, this is clearly very enterprisey and solution focused, but I would like to suggest a couple of amendments if I may?
Namespaces
We should make full use of namespaces. Make the structural tags be in a language specific namespace (to be referenced in every function spec, obviously) but change the in an out params to use the parameter name as the tag, namespaced to the function they're for, with a type attribute.
In memory message queues
Have all function invocations be marshaled as xml documents posted to an in memory message queue. Said documents should use a schema that validates the structure and a function specific schema to validate the types of arguments being passed. Namespace everything.
I reckon we could power a medium sided country if we could generate energy from the programmers despair.