I know this has been brought up before, but it's been a while so I'll bring it up again.
I have a need to run Nasal code at the same rate as the simulation. Currently, without modifying the source code for FlightGear, the only way to do this is to find a property updated at the right time in the simulation cycle and set a listener on it. From a code quality standpoint, this is less than ideal.
There is a thread on the forums discussing this at http://flightgear.org/forums/viewtopic.php?f=46&t=17069#p164697. In this thread, a couple of viable ideas are discussed. These ideas are: