The only effort I know of, for porting SIlverlight to Linux is Mono's Moonlight project. I am not sure if it has an API that fulfills your requirements

http://www.mono-project.com/Moonlight 

Thanks
Yasser Makram
My Blog: http://www.silverlightrecipes.com
Sr. Architect
Santeon Inc. Microsoft Silverlight Partner, Solution Provider

What kind of support you are looking for in terms of generating the content?   Any XML library would do.

And if you are brave enough, you do not even need an XML library, a bunch of print/println statements in your Java/Servlet or PHP file will do:

print "<Canvas>";

print "    <Rectangle ...";

 That kind of thing

Miguel 

 Your approach is good for some primitive apps or any prototypes...but not for serious apps. it's like reinventing the wheel.

Agreed, the above is a primitive way of doing things, but you were not clear on what you wanted initially.   Porting graphviz to generate XAML should be a piece of cake though.