Neodymium magnets are magnetized post forming and plating, or so claims this magnet manufacturer. If the magnet is permanently demagnetized, you could try to remagnetize it with a coil and high-current capacitor bank. There may be some interference from current induced in the aluminum, which may be addressable by using a longer current pulse.
First I would try heating the magnet in question to the temperature where aluminum melts, perhaps by pressing a ball of aluminum foil around the magnet and heating it in a cast iron pan. The foil will monitor the temperature, and touching/moving the magnet during the process will show you the demagnetization properties.
You are casting the aluminum around the magnet. If the aluminum is thin enough, it may be that the temperature of the magnet never reaches the critical temperature.
Since Aluminum had no grain structure (like wood), it may be best to cut the rod, create a cavity, polish the ends, glue it all together, and then turn and polish the rod on a lathe to remove any discontinuity from the cut. Carefully cut, polished, glued, and finished, the break in the aluminum rod could be nearly invisible.