The datum could be anywhere. Often on fixed wing aircraft it is the front bulkhead but it could be a position in space in front of the aircraft, maybe relating to a position on the jig when the aircraft was built. Helicopters often use the rotor mast.
The advantage of a forward datum is that all moments are positive. If you chose the main spar, say, then masses forward of the spar would have a negative moment, masses aft would have a positive moment.
On the MRJT the datum is defined as 540" forward of the front spar. It's not exactly on the point of the nose, it may be co-incidentally at the front pressure bulkhead but I don't think so, the maintenance manuals seem to show that being at STN178, 26" back from the datum.