Seems like a simple enough job. How we deal with this though depends on your hosting arrangement.
Most hosts will allow you to put files one level lower then the web root, but it all depends on what their policies are. From there, it's as simple as hooking up a script to your shopping cart to create the timed entry. From there it's up to you: you could simply give them the link in a web page, or respond with confirmation of payment. From personal experience I would suggest the latter - it gives you more leeway with people who auto-close payment pages while they're still loading.
I'm defiantly interested in this project, and I look forward to hearing from you!