How to access multipart parameters in custom SpringSecurityFilter using Grails

Question

I'm working on implementing a URL token based authentication scheme (using Grails) and have been working off of Glen's very helpful example:

http://blogs.bytecode.com.au/glen/2010/01/15/hacking-custom-authentication-providers-with-grails-spring-security.html

The problem I've run into, is some of my requests are multipart/form-data and I'm wondering what the best Grailsy way is to access the non-file parameters. I don't have access to the params object like I would in a controller but I'm wondering if they're a way I can inject that in and let Grails handle it or if I need to do it the hard way.