I have rebuilt motors without inserting a metal 'keeper' in place of the armature with no ill effects.
If you want to you can slide an iron bar in as you draw the armature out.
Warning, as soon as the armature leaves the bearing, the tendency is for it to be immediately attracted to the permanent magnets, care has to be taken not to chip or fracture the magnets.
I am not sure how your tech came up with high currents being possible with 5meg at 2000v, at 200v it will be the same or alot less, most likely.
Blowing the carbon out is a quick fix, but if the build up is high or the brushes worn right down, the motor should be disassembled and the brush holders completely cleaned or tracking can occur.
If the motors are controlled by analogue input drives, they can be replaced by easily available Advanced Motion or Copley Controls.