Tried looking into this, see also: https://github.com/FAForever/fa/pull/4218
It appears there are two solutions that are viable:
- (1) We remove the ability
- (2) A magical engine patch that manages to isolate harbingers somehow to let them move like support command units
If you bet for option (2) then chances are it remains like this forever (pun intended). Option (1) can be done right away with the next balance patch. I highly encourage option (1) for the sake of consistency - it is a nice idea, but if it doesn't work then you've got to adapt.