It really looks like you could use Identity Server for doing a quick little login and then bounce you into your site. I noticed a URL switch that was pretty promising:
First, the wrong answer: do not use the URL that you really want to go to. It won’t work for a ReturnUrl. Instead, use this for your ReturnUrl=%2fIdentityServer%2fissue%2fwsfed.
Then, once you are authenticated, the identity server will redirect you to the URL that you specified in the server config screen as the “Redirect URL”.