Reported by Imperator. With the new Mesophon buying options and fleet transfer abilities, you can apparently get combined clearspace-nebular fleets that act like one or the other.
Mesophon planets should probably only sell appropriate units for their zone.
Alternately or additionally, combined clearspace-nebular fleets could default to the propulsion type that is NOT appropriate to their zone in order to immobilize them. So a combined fleet in clearspace would be interpreted as a ramjet fleet but the same fleet formed in a nebula would be treated as having warp propulsion. When future cases arise where ships wind up in the wrong zone (e.g. through dark nebula duplication), this would stop them from doing anything mischievous.