I ended up creating a software solution that included break reminders but also uses your webcam to check your posture and remind you whenever you sit in a bad posture (or even a good one, since sitting rigidly in any posture for long periods is part of the problem).
It's not a substitute for exercise, but it is a simple, low-cost way to keep good posture in mind throughout the working day, without being overly intrusive. It's called Postureminder and there's a 30-day free download on my website, http://www.postureminder.co.uk
Phil