Best thing I can think of to cover humans would be to rewrite the urls using javascript. If they are all relative then it shouldn't be too tricky.
I'd note that mixing sites like this have some drawbacks, principally browsers don't like HTTPS pages with insecure HTTP requests involved. A better plan might be to make everything relative and only kick over to https mode once you need to hit the https section.
I can see a few ways to do this using IIS, but those options really depend on how much you can muck with the IIS configuration and what sorts of resources you have.