All calcs are based on Regice's 301 HP and 236 physical defense.
~100 Attack Careful Registeel's Iron Head does 51.83% - 61.79%... making Counter a OHKO if registeel takes of 171 HP (the average it would take off) and if Registeel takes SR damage. Regice's main counter is down and it can sweep easier now.
~252 Attack Jolly Choice Band Drapion does 47.84% - 56.15%... counter is an easy OHKO even w/out SR.
252 HP, 172 Attack Impish Thick Fat Hariyama does 58.47% - 69.77% with force palm... An easy 2HKO with counter followed by a Thunderbolt. Versions with brick break can only roll 86% max. Revenge versions attack first since you are using the -5 Priority move Counter while they are using a -4 Priority move.
It isn't an insane theory at all... there are many situations where a pokemon will be up against Regice and a quick counter can ruin a team member who is vital to their team. Not every team even has a fighting type like hitmonlee so it definately has its uses. Also, Hitmonlee is almost always choiced so if you get a rock polish while they switch in, they aren't going to want to risk overpredicting and going for another move because you are a threat now. Pair this thing up with a ghost type sweeper like sub-charge rotom or NP mismagius and you can have a dangerous strategy going on. Hitmontop's main purpose is to rapid spin, so if it sees a situation of weakness where you will switch, it will rapid spin instead of CCing, allowing you to get off an ice beam or go set up with your ghost.
Toxic, as I previously mentioned, basically assures Regice that it will beat Hypno and Grumpig. They will not be doing much with psychic because of your Base 200 sp. def, so that gives you time to stall them out and continue sweeping without having to explode on them, which is just a terrible option with regice's base 50 attack and necessitating the use of a -Def/Sp. Def nature.
There may be "other" options, but I was posting the ones I thought to be the best, so you don't have to use them, but they can prove their use easily. And when your focus blast misses and losses you the game, or your explosion does only 80% max to a Hypno (if your nature is neutral towards attack) with 56 defense EVs, don't whine because you will know that those moves aren't very effective on regice.
And Pizzaman - thanks for backing me up on this :)