If I understood your problem - you can live with having a single form on the master page (in which you place the button). Now in the content pages you can have your controls and make them send data.
Perhaps the only thing that you need to be careful about is the validation group for the controls.
In one of my projects I have a login section defined in the master page (with text boxes and a button, unless you log in you see those). In one of the content pages (using that master page) the users are allowed to register themselves (controls and sending data), hope your scenario is something similar