How to deal with a PHP parsing conflict within EE tags

I’m using the EE’s category/subcategory concept, but in a slightly different way than it was designed. I want to display categories in a select box, and when the user selects one, I use javascript’s ‘onselect’ method to redirect to a new url that includes the cat_url_title of that category and populates the subcategory select box, where I’m displaying ONLY the subcategories utilizing the unpublished EE plugin: pi.subcatgeories.php (http://ellislab.com/forums/viewthread/27220/). Why it’s unpublished, I don’t understand, since it’s rather useful in this scenario.

My problem lies in the fact that I need the template preference: ‘PHP Parsing Stage’ to be at ‘Input’ in order to include PHP code that sets the root_node value based off of the url segment, and I need the ‘PHP Parsing Stage’ to be at ‘Output’ in order to set the Category SELECTED option to the option that was selected, also based off of the url segment.

I can’t resolved this by breaking up the template into embedded templates since the PHP code to do both is within the same EE tag. What can I do ?

Please help - thx