Unfortunately, the 1-minute interval saving of motion events is buried deep within the algorithm and file structure used to save SD card events. It would take a complete rewrite of the firmware to change this and so I don’t think there’s really any chance of that happening.
That said, you might want to hop over to the post below and VOTE for it (button at the top of the page). If this gets implemented with a granularity smaller than 1-minute intervals, this might indirectly accomplish what you’re looking for.