www.endusersharepoint.com/.../

This post elsewhere is very helpful but there seems no way to edit the Add New Item url on the related  list's webpart  in Foundation 2010.

There is no easy way to resolve this issue.

I would recommend to do that through code. It needs a web part (as consumer) to accept web part connection and display the filtered data, and also provide a link which links to another web part page  with url parameter to add  new item.

In the web part page, a web part acquire the data from url parameter, and then put it into the dropdownlist control as default value.

This solution is far from good. However, to make the whole system flexible and easy to maintain, Microsoft had to keep the dependency among objects minimal, which caused this drawback.

This will solve it easily using JavaScript.

code.google.com/p/sp2010-related-list-prefill/

Let me know if it works and if you have any feedback! E-mail is on the site.