Passing params back to controller?

[Andrei]

Is there a way to pass parameters back to the controller? 

My situation: 

I have a {{form form_name}} tag which creates a form, collects and saves the data on :post. I would like to give a cookie acknowledging that this particular form was processed so that I can present additional information to whoever submitted the form. 

How can I pass a message from the form_tag back to the controller?

This is a weird situation where I need to assign a cookie but I only know about whether I should do so in a model instantantiated inside a liquid tag.

Thank you!