Just talking out of my ass here but...
Lower back pain during running can be linked to poor shoes, etc. but I think the most likely culprit is poor gait. The issue is that rather than having posture upright a lot of people run sort of hunched over. The net effect is that rather than transmitting the load to the hammies and glutes, the lower back is used for locomotion -- causing pain. Simplest fix is to find a running coach and ask him/her to look at you while you run. Try running but look ahead into the distance for as much of it as you can. If you are running and watching the ground in front of you (common) then you are hunched.
Reverse hypers are a great exercise if you know what you are doing but can mess you up if you don't. They are strictly a hamstring exercise and if you are using your lower back at any point during them, you are doing them wrong and should stop.
Also, check how your feet hit the ground. Are you landing on your heel? Should be mid-foot or fore-foot. Walking is a different operation than running. Other issues might play into this (e.g. flat feet on concrete with bad shoes).
(People who run indoors on machines should be very careful when moving outside as the weather warms up. Elliptical trainers will allow you to have horrific mechanics with little penalty (and I've known several people who promptly got back pain when they tried to switch to outdoor running). Treadmills teach you to shut off you hamstrings at precisely the point you need to engage them since the belt pulls your leg through the stride. One of my buddies who ran a lot inside (elliptical) decided to do a 5 k run for charity and ripped out a back muscle in the middle of it. The mileage was lower than what he normally did too. This last bit does not apply to you but might very well apply to your clients, so always ask them what their activity history is before setting out.)