In the mailing list there is a discussion ("Easy parseable, machine readable, input and output interface for Maxima") for a machine-readable output format for maxima. Actually maxima already provides such a thing for the maths-part of its output: MathML. The only thing that is missing is <annotation encoding="application/x-maxima">
tags or attributes for all objects that are output so that on drag-and-dropping a selected part of an equation the front-end into maxima knows how (mstring selected-part-of-the-equation) looks like. Perhaps we could add that and thereafter tell users who want machine-parsable output for front-ends that this is an option. MathML is only an option for front-ends that want to display maxima's maths, though...