Can Monks wear helmets? Specifically, can monks where a magic helmet without being considered to be "wearing armor"?

There are a number of magical helmets that provide no AC but have other rider effects, from the common Dread Helm to the very rare Helm of Brilliance. Nowhere are these called out as "armor", but many might consider them to be armor.

Though, it is universally understood that Bracers of Defense can be worn by Monks, and the description even calls out that the wearer cannot be wearing armor, but bracers could very easily be considered just as much armor as a helmet.

Additionally the Spelljamming Helm and Helm of Disjunction require attunement by spellcasters, with two of the three HoD classes get no armor proficiency (would wearing these make them unable to cast spells?).

It seems to me that helmets are not actually armor in the D&D world, and thus a Monk could wear one without losing access to any of the class features.

Helmets by themselves are not armor. They don’t appear on the armor tables in the PHB. They may be part of a set of armor, but themselves wouldn’t count when considering monk features that require not wearing armor.