So, it appears that any worn headgear (hats, helmets, glasses, wraps, masks, etc.) receives no damage at all in combat unless its equipment slot mask includes either the "head" or "hair" slot.

Probably a leftover from Oblivion which only had those two slots usable without tricks.

Is there any kind of fix or workaround for this?

You could run a script (token or quest) that monitor's the wearer's health. Then when damage is done, you could modify the object's health accordingly with DO (damage object).

Well, that's not going to be hugely reliable, as the PC's health changes aren't precisely correlated to their damage taken thanks to the fact most PCs will be chugging stimpacks and food all the time, regularly get poisoned or set on fire and the like. The only alternative I can think of is to actually track the Hair slot health and just copy the damage across. One note on that- for no obvious reason, the T-51b helms don't take up the hair slot.

Not surprising about the t-51b helmet since it uses the head slot like the Ghoul mask. Presumably, head slot overrides hair slot and it should receive damage as per normal.

A script to monitor damage to the item in the head and/or hair slot and apply the same percentage of damage to any other head items is probably best. I was hoping someone else had already done it and integrated it into FWE or something.