Well I was doing a lil research just now and I have to revise my original statement, I feel that the helm might actually drop at a lower rate than the other 2 rare pieces.
This is the break down by common and rare pieces.
Honor Plate:
Common Pieces: arms, gloves, gorget
Rare Pieces: tunic, legs, helm
This is the list of of peerless and monsters that drop which set piece:
Irk
* Plate of Honor
* Gloves
Lady Lissith
* Plate of Honor
* Sleeves
* Gorget
Lady Melisande
* Plate of Honor
* Leggings
Lady Sabrix
* Plate of Honor
* Sleeves
Miasma
* Plate of Honor
* Arms
* Gorget
Monstrous Interred Grizzle
* Plate of Honor
* Gorget
Swoop
* Plate of Honor
* Gloves
* Gorgot
* Arms
Travesty
* Plate of Honor
* Gloves
* Tunic
* Sleeves
First you may notice there are a number of names missing, those not listed have not been reported to drop plate of honor pieces. Secondly you may notice there is no helm listed, the possibility is that no one has reported it as having dropped since which is quite likely since there are full suits out there. I would also assume that the drop rate on the particular piece has been much lower than the rest if no one has reported it, but all the other pieces have had the drop submitted.