Is it possible to adjust the base_url setting from conf_global during page rendering? I'd like to offer the ability for members to enable HTTPS at their discretion, but since we have a few still on Windows XP and we use CloudFlare, I can't enable it globally.

If they're still on XP. It's time for them to get with the times and upgrade. As as site owner you wouldn't have to sacrifice security for their lack of security lol