I'd like to test some turret designs against various enemies in the R&D chamber to see how well they can defend my base when I'm not around. However, I have to be in a tech myself and that can easily distract the enemies. Is there any way by which I could make myself immaterial or invisible? Would a high-altitude hovercraft help?
Not really.
Note that you have to be close to the enemy for it to actually fire back. If you're not, the game itself freezes it. To see for yourself, just spawn in an enemy airship, fly it up really high, and watch it start to drop. Notice that when you get far enough away it just freezes in midair. That's cuz the game engine no longer bothers to calculate any physics for it, and the AI scripts just stop running as well, so it doesn't fire back.
This is also why long range artillery in campaign is so devastatingly effective if you really want to cheese using it. You can take as long as you want to adjust your aim, the enemy ain't goin anywhere.
The best you really can do in the current system is just place a cab on the ground behind your tech, so the enemy will always target the other tech first.
When I was reenacting naval battles for the

I would just sit in a cab nestled behind the bridge of a friendly ship, well inside shield range. Only way i die and thus lose my camera angle is when my ship dies.