Question about buttons and stuff, I guess it would work here: Is evolving hard coded to certain units? Because if not, then you could 'simulate' cloaking with evolving.
Marine-->Click Evolve Button. The Marine evolves into a permanently cloaked Marine who doesn't have the Guard action as it's default AI, so it won't auto attack. Then the Cloaked Marine could evolve back to a Regular Marine.
But I have a dark feeling theres a catch here somewhere.
Hmmm... I know Dark_Soul74 was researching that and I'm sure he got units to evolve, but I can't tell you from first hand experience what to look out for when you try it.
That idea works great, but during the morph it will use the zerg egg or cocoon graphic. That part is the only hard coded thing.