Yes they have been. Still no reason to be wastfull (because while the engine may not be limited, your hardware might be). Still, one or two duplicated objects are not going to make a difference.
OTH, I would have sworn the invincibility flag CAN be set with script, which you could trigger on level entry. This would make the player utterly invulnerable though, not just have infinite health.
Another thing you could do is look at the dohit events, and nullify damage if in said level. That way player could still take hits, just never die. That's a pretty involved thing to do though if you are a novice at script, and probably more than you want to deal with for a bonus level.
DC