Limit search to custom post type using hidden form fields

A new site I am developing features a custom post type in the form of a Document Library.

I want the main site search to remain (and be powered by relevanssi), but I would also like to be able to have a second search field on the Document Library page that would ONLY search the Document Library posts.

I see from the Relevanssi settings page that you can dynamically limit a search to particular tags & categories by adding the code <input type=’hidden’ name=’cats’ value=’list of cats and tags’ /> is there a similar method to limit a search to a particular post type?

Many thanks for any help in advance.

T

http://wordpress.org/plugins/relevanssi/