[boost] date_time using CPU performance counter

I need high precision under Windows XP but don't need long-term date accuracy. To that end, I think it would be useful to have a microsec_clock implemented in terms of the Windows QueryPerformanceCounter API (which wraps a CPU register query) instead of the GetSystemTimeAsFileTime API (which is only good to 15 msecs and almost guarantees a context switch). Has anyone attempted this? How hard is it to code up a new clock source to fit into the Boost date_time design?