Secure Pages Domain Access for Drupal 7
You can actually just leave the base URL and Secure URL blank to not have it redirect to an inappropriate subdomain than you're on currently.

To restate my problem, I had multiple subdomains sharing one database where the Secure Pages module was implemented. If I put down a base and secure URL using one subdomain, I would get redirected to that instance from other subdomains not listed there when https was needed for that subdomain. 

This is an undocumented solution as far as I'm aware, so I hope it helps someone else out.