The PG says:
So you don't ignore the stats until after the figure has been MCed. At the time of the attack, the bonus is not ignored.

Yes, it is answered in post #2. I highlighted the portion that is relevant.

So the loophole essentially being that you can mind control a character who picked up a relic and use the relics affects for that character since it wasn't costed with that character during construction of the team.

I suppose you could call that a "loophole" but it would require that your opponent is playing a relic and has picked it up and you hit it with mind control and the relic is useful to you. Or you could argue some kind of mole man thing but again, you would have to sacrifice your resource to put a relic on your force, successfully pick it up etc...Really other than mole man I can't think of a situation where the distinction regarding force construction would matter.

I wouldn't call that a loophole. It's just how the rules work. Most Relics aren't guaranteed to be picked up.

or play the belt, and drop items so you can pick them up as relics

Yea but again, without mole man, since he is the only option to be MCing your own figures with relics, you have to rely on your opponent playing a relic which is highly highly unlikely since they are so underpowered.

Yeah, its a rather weak loophole, but to know the option is there is pretty sweet. Using the compel feat could get any character to use the items twice for instance. Like getting the batarangs (only a 50/50 chance to grab) and having them incap some characters and then immediately doing it again through Mind control. Of course this is simple thinking here. You can really get creative with some relic/MC interactions.

Except the gem isn't attached diring force construction it's at the beginnin if the game had nothing to do with Mind Control and was about the MAY PART of attaching the gem