Using threat won't work since there are abilities that reduce hate (Guard) and I'm pretty sure there's healing abilities that generate less or no hate than the actual heal amount. Basically, threat is not going to be very accurate to try to figure out actual heals.
Yeah, as Wugan said, I wouldn't be too quick to dismiss it. I believe torparse is using threat as a way to compute effective healing, and it seems to be working rather well. The difficulty just comes from the talents that reduce threat as well as Guard, but I believe you can readily discern whether or not the talents are in effect as well as whether or not Guard is in effect simply by observing the heal numbers. There will be enough "full heals" that you'd be able to determine the modifier for each individual.