Well, really two different scenarios to go with this; the intended training audience would have their sensors, including active probes programmed to read the holo-projections as real. Uninvited guests active probes would not. Fancy sensors might not be fool-able and the pilots would need to play along.
I'd say let your holo-projections do whatever a normal unit could do, except effective physical attacks. It's not hard-light afterall.
What if there was a back-door override that had been kept in the programming for centuries and no one ever thought about addressing or cutting it out, or when they did it caused the system to fail? I give them a little fluff about how their targeting systems seem to cycle unusually as they approach the field.
Before I got this information, I was going to treat them like the Beginner Box in a way. Basically, any through-Armor hit would cause the hologram to disappear as a kill. So wiping out the Armor normally or getting a TAC.
If anyone asks about it, I'd have them do a PSR to try and figure it out. Maybe give them a bonus for the same conditions as The Fall of Terra's standard.
Part of the story was that the Clanners scored far higher than the Spheroid commander would like, which means they engaged them pretty effectively and thoroughly instead of focusing on their original prey.
Of course, they could have just considered it another challenge and ran with it, too. Clanners be weird, after all.