Its possible that there's some weird interaction where you need to have an OH equipped for CP to proc. Can you try testing with an OH equipped, but turn to disable autoattacks and generate MG procs via SS (or equivalent) spam?

Oddly enough, the talent calculator linked by MMO-C (wowtal.com) shows that you should be receiving CP procs from your MG hits.

Combat Potency: Gives your off-hand melee attacks and Main Gauche attacks a 20% chance to generate 15 Energy.
Main Gauche: Your main hand attacks have a 16% chance to grant you an attack that deals damage equal to 100% of a main hand attack. Each point of Mastery increases the chance by an additional 2%.

So I think it's likely that it is still intended to proc CP, but just broken or NYI.

Seems to be bugged or NYI, the updated patch notes state

•Main Gauche (Mastery) now provides a chance to deal an attack for 100% of main-hand damage. This attack can trigger Combat Potency.

Public Test Realm Patch 4.0.6 Notes - Updated Jan. 11 - World of Warcraft

Edit, I also found this hidden down the bottom,

•Synapse Springs now increase Agility, Strength, or Intellect (whichever is highest for the character).

It hasn't been noted yet, but:

Alchemist's Stones for Agility, Strength, and Intellect have been added.

Which means there's a level 359 static Agi trinket being added. Stats should be similar to:

+ 380 Agiliy
+ Socket/Bonus
Equip: Increases your mastery rating by 213.
Equip: Increases the effect that healing and mana potions have on the wearer by 40%. This effect does not stack.

Also:

Left Eye of Rajh: Chance of triggering increased.

Stats are out:

+301 Agility
Red socket with +10 Agility bonus
Equip: Increases your critical strike rating by 194.
Equip: Increases the effect that healing and mana potions have on the wearer by 40%. This effect does not stack.

Please Delete Post Above

Patch 4.0.6 Notes Update - January 11



* Blind now has a PvP duration of 8 seconds.
* Crippling Poison now has a PvP duration of 8 seconds.
* Sap now has a PvP duration of 8 seconds.
* Stealth is no longer broken by enemies using Demoralizing Shout or Demoralizing Roar.


Assassination

* Deadly Momentum critical strike bonus now lasts for 15 seconds, up from 10.


Combat

* Main Gauche (Mastery) now provides a chance to deal an attack for 100% of main-hand damage. This attack can trigger Combat Potency.
* Restless Blades now also reduces the cooldown of Redirect.
* Vitality now increases attack power by 25%, up from 20%.


Subtlety

* Executioner (Mastery) has been increased from 2% to 2.5% per point.
* Find Weakness now grants 35/70% armor reduction, up from 25/50%.
* Preparation no longer resets the cooldown of Evasion.
* Sanguinary Vein now gives a rogue a 50/100% chance for their own Bleed effects to not break their Gouge.
* Shadowstep's cooldown has been increased to 24 seconds, up from 20 seconds.
* Sinister Calling now grants 30% Agility, up from 25%, and increases Backstab and Hemorrhage damage by an additional 40%, up from 25%.

Another day, another batch of patch notes:

Garrotte energy cost being reduced to 45, Cooldown Reduction on Cloak of Shadows from Elusiveness Nerfed (10/20, down from 15/30), and Blade Flurry now acts as a toggle (a nice quality of life change, combat rogues will no longer need a /cancelaura macro)

Would this apply to the other poisons and abilities it currently does not proc from?

Am I reading the Blade Flurry change now to mean that if there was a fight where two MOBs were standing side by side for 5 mins, we can now have Blade Flurry active the entire time (not that many would due to energy penalty)?

Under current understanding of the ability: yes, you can keep it up for the entire duration of a fight, regardless of length. And, energy penalty or no energy penalty, you very much should provided both targets legitimately need to be DPSed, as the damage you gain from duplicating 2/3 of your damage massively outweighs what you lose from a small energy penalty.

And if you think this sounds somewhat imbalanced and likely to be nerfed at some point: I agree with you.

I dunno, it's quite good in heroics to be sure, and for clearing trash. But for actual boss encounters, I only see it as "imbalanced" on Halfus. No other boss really benefits from multi-target killing for extended periods of time. It is useful in some parts (adds on Conclave, last 25% of Cho'Gall, maybe for phase 2 of Al'akir) but certainly not imbalanced in these cases.