Hi,
I have written an article in my Blog about this, it is written in german, but following the link in the article you can copy the code for a server-side Javascript library and in my article there is the code for an additional method, named getURLParam.
Look here: http://mnicklisch.wordpress.com/tag/cgivariables/
Using this library, you can easily draw values from the query string. You can set this value as the default parameter of the field in the XPage, but you have to remove the default value from the form itself.
Hope this helps.