Set session.save_path in config.php?

I had an old Perch installation that needed updating, so I pulled it down to dev, replaced core, and pushed it back to live, at which point I couldn't log in. After some troubleshooting, I realized that I had added session.save_path to `PerchSession.class.php' which was overwritten when I updated. Now, I should have made a note of this but of course I didn't, so I had to troubleshoot all over again.

All that to ask, is there a recommended method for changing session.save_path without modifying a file inside core? I searched the docs but didn't see anything. If there isn't, maybe it could be added as an option for config.php?