You can achieve this with the mgdraw.f user routine, using the BXDRAW entry. This code is run each time a particle crosses a region boundary. Here, you can filter out the crossings you are interested in (location, particle type, energy, etc.) and write the crossing coordinates, direction, particle type, energy, etc., into a file.
You can find an example here:
You may want to look at the source_newgen.f routine to see the format used there for phase-space sampling.