Hi John,
We are passing the return url to paypal so that it returns to our PDT handler which processes the order and then redirects to the order confirmation page. I don't think a 301 redirect would be a good idea because that is a permanent redirect. Keep in mind that the link to the order detail page also appears in the user profile under order history.
We do have a plugin system that allows developers to implement an order completed handler to execute code after the payment has cleared. However I don't think it would be a good idea to put code there that forces a redirect because that can also fire from the IPN handler which happens by a server to server post from paypal that doesn't involve the user or his web browser.
About the only easy thing I can think of to accommodate you is I could add a web.config setting where you could put an override url and if that setting is a string we could return that instead of the url to the order detail. Maybe that would meet your needs but it seems like a strange solution to me since that ThankYou page is going to be the same for all users and not really have any order detail at all.
I think a better solution is to either implement an order completed event handler that sends a an extra email with a request for more info and a link to the page where they can enter the info, or using a custom message template for the main order confirmation email that includes a link to the page where you collect the additional info.
Best,
Joe