I want to test input validation of all WebForm fields for any given field, and not have to breakpoint and manually change values for each of the fields, and then be able to view the results pages.
Fiddler can modify any aspect of any request or response, either manually using breakpoints/inspectors, or automatically using the FiddlerScript engine (Rules > Customize Rules).
Fiddler can also generate new requests based on existing requests; for instance, drag a request from the Web Sessions list to the Composer tab, make any modifications you like, and click Execute.
So, at a high-level, Fiddler can definitely accomplish the task you've described, but the exact procedure may vary depending on what exactly you're trying to accomplish and what sort of automation you're looking for.