Local coordinates in a volume+external event loop

Hi, I would like to retrieve the local coordinates of a hit inside a specific volume (defined through TGeo), ie the coordinates should be extending from the center of the volume (0, 0, 0) up to its boundaries (for example length and radius of a cylinder as maximum values). Is the gGeoManager->MasterToLocal the correct and fastest way to do this? (I plan to have this call in Stepping function so it should be as efficient as possible).
Another question, there is a TVirtualMC::ProcessEvent function. Is this called at some point in the simulation? The reason I am asking is that I would like to have an external event loop for the simulation if possible but as far as I can see for example in TGeant4::ProcessRun(int) G4RunManager::BeamOn(int) is called directly (at least last time I checked some time ago). Is there a way to have an externally-driven event loop bypassing this then? Thanks,
filimon