So, does such an API sound interesting? Why? Why not? What caveats would you anticipate? I've thought of several, but I want to make sure I know the risks. Anyone have a suggestion for an API or utility that is similar in Perl or another language? (That is, metaconversion tools, not actual converters like XSLT or ImageMagick.)

Well, print filters are one mechanism to control document conversion (/etc/printcap and magicfilter for example). But these specify all paths directly, multi source / single target. Yours sounds like a nice general document transformation API (probably easily implemented as a graph) that would be useful and easy to update.