It’s working with swords.
Thrall in Silent Legion = 880 Armor
40 Strength
39 Accuracy
Telith’s Lament - 53 Damage, 10% Armor Pen
Light Attack: 32 Damage
Heavy Attack: 37 Damage
Telith’s Sorrow - 66 Damage, 15% Armor Pen
Light Attack: 45 Damage
Heavy Attack: 77 Damage
40 Strength
40 Accuracy
Telith’s Lament - 53 Damage, 10% Armor Pen
Light Attack: 37 Damage
Heavy Attack: 42 Damage
Telith’s Sorrow - 66 Damage, 15% Armor Pen
Light Attack: 53 Damage
Heavy Attack: 91 Damage
Interestingly, Brutal Strikes does not seem to work on the first heavy attack with a one-handed sword, but it does apply to the second heavy attack.
If you apply a Spiked attachment to Telith’s Sorrow, the HA damage is bumped up to 130 because it now has 45% armor pen with Crevice instead of 22.5%.
Edit::
I created a new thread with all the test results.