Using values from the admin screen form in an action

[Waitak]

My use case requires that I modify one of the fields in the admin form, and then invoke an action. Basically, I need a function signature like that of ModelAdmin.save_model, which gives me access to the form in addition to the object. Using a POST (as discussed in #16) to do so seems like a workaround, but I'm not sure how to do that either. Any words of wisdom?