You have to use the txi trick, which I've been trying to pull off with some heads I am currently working on (but have run into the reflective over transparency issue).
You essentially have your model referencing the plt, with separate nodes pointing to a fake tga/dds that has an associated txi. That txi then points to an alternate tga/dds.
Using PHOD's Neeshka head as reference (all in the same folder):
You have the head .mdl and it's associated .plt. Within the head model is a separate node trimesh for the eyes; bitmap reference "neeshka_h1".
neeshka_h1.tga is effectively a blank tga.
neeshka_h1.txi with the internal reference "bumpreplacementtexture neeshkaeye". This points to:
neshkaeye.tga/dds
With all five files in the same folder, the head inherits the plt while the eye nodes inherit the neeshkaeye texture.
I'm not sure the structure of the desired armor model, but the parts you want to inherit the tga over plt need to be separate nodes and I don't know that can be done outside a modelling program. From there it could get a bit complicated if you have more than one part that you want to inherit one of a number of tga files, resulting in the list of files going from five to 8, 11, etc with each reference.
I circumvented this in what I am working on by having a head node and a separate node for the eyes and hair adornments together (instead of two separate), but that is because I took three separate tga files and combined them into one, having to adjust the texture mapping. The problem here is your admission to not being versed in going that far.
