Customizing armor will be difficult depending on the style and century, you will not be able to fit a late 15th century breastplate over a 16th century gorget for example, or use an Armet with a gothic bevor, even then there are limitations with how cuirasses change throughout the ages. It would require alot of.. messing around to get anything performing right and looking right. The only piece of armor exempt from this is Gauntlets, almost any type will work with any vambrace or harness, minus late gothic elbow gauntlets.
HOWEVER, It might be feasible if you were to do one era. and if so, I imagine the armor customizing mod would benefit the most with a system like this :
Head (an Armet for example)
Neck (Wrapper plate for said Armet.)
Torso (Cuirass)
Arms (Pauldrons/Spaulders and Rear canon, floating or articulated elbow, vambrace)
Hands (Gauntlets)
Legs (Entire leg harness minus the feet)
Feet (Boots, shoes, or sabatons)
We'll see when bannerlord is out and mods are being made.
edit : attached two photos, of two very different styles. first is a late german harness of the 16th century (1525 approximately) with a close helmet. Second is a gothic harness of the 15th century, Sallet is a reproduction and the bevor might be too. If you pay close attention to the neck area of the armors you can see the differences with each one. even the waist is different. mixmatching will be difficult.