The feature would have to come from the PSU, not a fan controller unless you were to have a dedicated power supply (not necessarily a PSU) to supply the fans with power after the PSU has shut off.
I have heard of this feature for the fans in certain PSU's, it's not for fans outside of the PSU though. I have not heard of any PSU's with that feature on molex connectors which are dedicated to fan use, which is what you would be looking at with this feature.
It is interesting, the problem is keeping people from plugging something other than fans into the connectors that supported the feature your after.
The feature would be easy enough to implement. A temperature sensor (RTD, or thermistor) coming out with wire bundle, and some dedicated molex connectors for your fan controller or the fans themselves. The problem would be that it would be unlikely that the temperature at which the fans shut off would be user adjustable. Unless of course the PSU came with it's own fan controller that had some sort of feedback control (digital or analog) to the PSU so that you could set the temperature where the fans shut off. All in all not a hard feature to implement, there just has to be a market for it. The other thing is that it goes beyond the ATX specs and it would likely be proprietary, something that most manufacturers avoid.