Hi Broderick --
I might try using a combination of a hidden field that autofills based off of the URL parameter and Visibility Rules on the field that needs to change. The idea being the URL parameters are actually setting the hidden field, which in turn updates the picklist order/values on the visible field. There may be a more elegant way to do this, but that's where I'd start.
Like this: MktoForms2 :: Fill from Query
Note the use of the FormsPlus Util lib (the base of my other FormsPlus libs). It's the <script id="teknklFormsPlus-0.2.0-Util">. Download and include that on your pages after the <script> that loads forms2.min.js.