I have prepared a simple patch (only basic functionality, no integration in g_object style programming, parameter passing with a global variable) to limit the number for MAX_CONSOLE consolekit can use. It is more a feature request than a solution.
You can pass a parameter --limit-vts N or -l N to the console-kit-deamon to limit the maximal number of VTs monitored. The patch reduced the amount of threads spawned. As I am using this only on a single user machine nothing else is tested.