First, you could clone the bizform webpart, and create an on after save event, that will create the registration. As far as what registration to place the person under, you can create a field on the page type that you will be placing the form on, which allows you to select the event that you want the registration to take place. So for each page of your page type, you will have a field that is a dropdown list or something of those lines where you choose the event, then the custom form webpart looks at that property on the page, and registers the attendee under the appropriate event based on the selected item on your page inside of the onaftersave event built into forms.