This isn't on the critical path, but it'd be a nice capability to have. Hopefully someone will fund us to do it. I did some hacking and got it to work:
It's all hard coded, but there's probably a wee bit of code that might be useful. For some reason svn isn't working for me at the moment, so I can't throw the diff up. But if anyone wants to see it just bug me.
Ideally the value to use as the extrude could be a filter, so you could just use an attribute, or some kind of function/equation, that even the user would be able to pass in to the server, so we can have dynamic extrudes. It should perhaps be part of the SLD. See http://trac.osgeo.org/mapguide/wiki/MapGuideRfc16 for how mapguide approached the problem.