just make 2 holes on the door:
1 on the lower side as close to the floor as you can
2 on the high side as close to the ceiling as you can
Do you pride yourself on how unhelpful you are? Cutting holes in the door is not easier than changing a setting on the router.. Please stop posting to this question, as you don't know what you are talking about. I am looking for an answer to my specific question, not a diversion from my question. Most likely the answer will come from someone at Mikrotik who knows about the hardware.
Well, he may not have come out and said in one sentence: "this isn't possible with ROS - sorry" - but I believe this thread has already given strong indications that underclocking doesn't result in less heat by the router, so apparently that option is out. Your ideal solution apparently isn't doable, regardless of what Mikrotik should or shouldn't do, or what Tile CPU should or shouldn't do. The fact of the matter is what it does or doesn't do.... which apparently doesn't include an option to generate less heat.
You also don't want to re-locate equipment to a more well-cooled/ventilated area (understandable) - so your options remain:
under-power some other device (you yourself said that it's the combined heat output that's hurting you), use another router that generates less heat, or else find a way to dissipate the heat. Unfortunately, Chechito doesn't have a magic ice wand to wave over the CCR - I'm sure he would have done so for you if he could, because he is a very helpful member of this community.
Your dislike of the facts doesn't mean that Chechito was "unhelpful." Again, he didn't outright say "cannot be done", but his earlier replies certainly gave ME, an un-involved 3rd party, the impression that the heat output isn't really something that can be reduced much, given the platform. Don't shoot the messenger - especially when he's actually giving suggestions on how to deal with the situation, given the fact that the router will belch out however many BTUs it currently does, give or take a few, regardless of its workload and clock settings.
Take it up with support for a completely definitive answer if that's what you want, but from my position on the sidelines, I will say that it sounds like time for plan B - deal with the heat by some other means than underclocking the CCR.