This area is a bit of a mess in the Pathfinder rules, and they've gone back and forth on how it's intended to work since the Adventurer's Armory came out (originally introducing some of these), and the unchained monk makes some of the previous FAQ rulings wrong/irrelevant.
Important things to note.
Unchained monks are proficient with any weapon that has the monk special weapon quality, but not similar seeming weapons. Gauntlet - no, cestus - yes, brass knuckles - yes, spiked gauntlet - no, rope gauntlet - no, knuckle axe - yes. If a weapon has the monk special weapon ability then the unchained monk is proficient in it and can use it as part of a flurry of blows. (Note - this is different from the normal monk, which does not gain proficiency in a weapon just because it has the monk special weapon ability.) You may already know this but I wanted to say it out loud.
The weapon damage replaces the unarmed damage. These are weapons just like any other weapon; nothing about these says they're different from e.g. a longsword. You don't add your unarmed attack damage to a longsword attack and you don't add it to a cestus either. Yes, this means your weapon damage would suck compared to unarmed damage at higher levels, but you can more easily enchant the weapons, so you have use cases for both. A monk in one of my games carried around some cold iron brass knuckles for all his low levels so he could smack demons and such until his ki strike let him do it with his unarmed attack. Amulets of mighty fists and bodywraps of mighty strikes are more expensive than enchanting weapons (and you tend not to find artifacts of them!).
While both the cestus and the brass knuckles have different special notes about how now your unarmed attacks deal lethal damage (brass knuckles = lethal, cestus = lethal and you threaten since you're 'armed' and do P/B damage) that doesn't override their damage entry in the weapon table. It just means people using them get part or all of Improved Unarmed Strike basically for free.
Note that in the first printing of Adventurer's Armory and early printings of the APG, the brass knuckles had verbiage saying a monk could use them with their unarmed strike damage and they were in an "unarmed" section in the weapon tables. This was specifically errata'ed and rolled back, you can read all the people complaining about the "nerf" to SKR on the Paizo forums here. The version in Ultimate Equipment is definitive and "official" (especially for Society) and the d20pfsrd uses its verbiage. So no unarmed damage with any weapons - armed != unarmed, by definition.
You can of course do it anyway... It's an additional buff, bringing the monk to "magic weapon parity," but since you can't really use any of the extant monk archetypes with the unchained monk, it would make it something more distinct, and you wouldn't have to worry about too many unexpected stacking things.