I have added another type to the LivePathEffect discussion on SVG representation. See http://wiki.inkscape.org/wiki/index.php/LivePathEffects_Discussion.
FIRST: <inkscape:path-effect xlink:href="x-inkscape-plugin:org.inkscape.effect.skeletal" id="path-effect2"> <inkscape:param name="skeleton" type="path" value="M 0,0 L 2,2 L 4,0" /> </inkscape:path-effect>
SECOND: <inkscape:path-effect id="path-effect2" effect="skeletal" skeleton="M 0,0 L 2,2 L 4,0" />
You should be able to infer the type from the specified effect and parameter name. So for the first one, there may be strange bugs when the parametertype is entered wrong! And what if there are two children with name="blah"? While I was coding for some reason I thought there was too much freedom. That's why I came up with the second one. It cleans the SVG up a little aswell. Also, xlink:href="x-inkscape-plugin:org.inkscape.effect.foo" seems overly verbose. The second proposal has a minimal effect specification (probably too minimal).
Thanks for all your input and comments. In the meanwhile I will start implementing the second one. If we want to go to the first or some combination or something completely different, at least I have the coding experience of the 2nd one :)
Cheers, Johan