Depends on what you mean by hidden. As in, not show the button to produce/morph/upgrade until they have the requirements? Not in MegaGlest (GAE's subfactions only). At the best, you can give them the old fashion requirements and make the unit a mystery image (ie, show a question mark for the button instead of an actual image of the unit) to try and fake it a bit, but that's still awfully rough.
