The answer was "there is no dedicated place", no "there is no place". The "issue" with FFA reports is that the format is not intended to be competitive nor ranked, so it simply uses Random Doubles Battles sets.
If your report is about doubles-only stuff like Follow Me, Coaching, or Ally Switch, then the answer is simply: "we know that the format isn't optimized, but that's something that isn't on our current plans or priorities, as it can't even be ranked because of policy and technical difficulties that go beyond Random Battles' scope". This includes moves failing when aimed to an empty spot, or other doubles hereditary shenanigans like Friend Guard, as they are also known and part of FFA being unoptimized for real competition.
If your report is about people not getting punished for forfeiting mid-battle, teaming up, or taking time to play, that's also beyond our scope, as the format isn't meant to be competitive so none of those are regulated, frowned upon, nor punished. Only people that brag about timer stalling can be reported, as with any other kind of battle.
Now, if your report isn't about any of this stuff, I'm actually curious to see what is it about, since I've only saw reports of those mentioned topics. Feel free to post it here and I'll try my best to clarify what's going on, if it's in my power and knowledge.
If you are unsure of posting, feel free to reach me or another staff member of the Random Battles room on the simulator.