Not really, you cannot really pass values to other html forms - I have used IWP to call PHP pages and have them run actions such as sending emails or even uploading files to IWP. You are limited as to what you can do with IWP as you have automatically generated template pages.
I encourage many people to work with custom web publishing, not only Instant Web publishing. Many times you reach limitations and work harder trying to find a work around then devejust learning custom web publishing where many of the difficult things with IWP are simple
But then again, some people truly enjoy the challenge of pushing iwp to its limits, and it is always fascinating to see their innovation and creativity