Hiya David,
Well as I said, there is no connection that can be made with the form that can direct you anywhere at all other than whatever page you have it setup to in the form for the redirect. You cannot have it redirect to the payment processor as this has to be done via the checkout as it normally would be in the cart system.
What I suggest is to add a form to the details page (you can copy and paste the code to each of your products that needs the form pretty easily). The form would be submitted separate from the purchase since the submit of the form would have no bearing on the ordering process at all, no connection there between them. You would probably have to figure out how to redirect them to their order or a page that wouldn't get them messed up in their ordering process.
That's about the best I can think of, and that's what many people are doing to battle this issue for now. Not a perfect solution, but a doable one.
Although I don't include customization features on my site, you can check out my shop and in most of the African products you'll see a form for asking if the customer wants to be notified when something is restocked. Just an example of how a form can be added to the product page:
http://accessoriesoftheworld.com/shop/v … ductid=712
My form just redirects them back to the home page, haven't worked out how to keep them on the same page they are on yet lol. Anyways, just an example on how a form is added to the details page.