What you propose seems completely reasonable to me. You will notice that some of the interpreters are listed explicitly in the following Inkscape source file. I don't remember if something has to be done here to make what you intend work properly.
http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/view/head:/src/exte... http://bazaar.launchpad.net/%7Einkscape.dev/inkscape/trunk/view/head:/src/extension/implementation/script.cpp
Update: I tried Inkscape 0.47 instead of 0.48 now, and here an error message is briefly displayed in the console window (translated):
Error: Main class C:\Users\USER\AppData\Local\Temp\ink_ext_XXXXXX.svgMEMVCW couldn't be found or loaded
Does that ring a bell with anyone?
To my eye that looks like java is successfully invoked, but the command line is inappropriate. Java is attempting to find your program inside of the temporary SVG file Inkscape created for you to process. I would recommend you do some additional investigation in to what command line Inkscape is using to invoke java.
Aaron Spike