The trick Jon's using in acs-person/www/add-edit.tcl is to define the key as an optional variable in ad_page_contract, as well as the key for ad_form. Then you can check for its existence to tell whether or not you're building an edit form or new data form before calling ad_form.
I've thought about adding request blocks similar to the submit blocks but I'm concerned about ad_form becoming overly complex and therefore hard to learn to use. It's quite complex already.
Also ... for the example you're describing - and Jon's example, too - the action (set a page title, set a context bar) doesn't logically belong with the form building code. ad_form is building a form within a page, it makes sense that you'd have page-building code generating template information that's not part of the form outside the ad_form call.
But ... I'm open to creative ideas as to how it can be extended to make it easier to use!
