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So I'm envisioning a Script Include I have to invoke from a client script, passing it arguments somehow, and then getting back an XML hunk that I have to parse in a client script.
How, round two
I've been working on the scripting. I had a working Background Script. Turned it into a Script Include. Then made a Client Script, using ClientAjax to invoke the Script Include with arguments. So far, it seems that the Client Script isn't successfully contacting the Script Include; something is going wrong with the communications.
There is a sample Hello World script that demonstrates the fundamentals of argument and return value handoff. I should try to make that work, and if it does, I should gradually build on that communications framework to see if I can isolate the problem.