I believe the below feat will allow a monk to increase her options for weapons in the game while not forcing them to give up the flurry of blows ability.
MARTIAL DEDICATION
Focused training with a specific weapon has taught you how to use it in tandem with your martial training.
Prerequisites: Flurry of blows ability, Weapon Focus (light or one-handed weapon), proficiency with weapon (light or one-handed weapon), base attack bonus +4
Benefit: You can make flurry of blow attacks with the selected light or one-handed weapon as your primary weapon. This feat does not grant the ability to use ki abilities through the weapon.
Normal: When performing a flurry of blows, you may only attack with unarmed strikes or with special monk weapons (kama, nunchaku, quarterstaff, sai, shuriken, and siangham).
Special: You can gain this feat multiple times. Each time you take the feat, it applies to a new type of weapon.
I like this, it seems quite balanced. I might drop the Weapon Focus prerequisite though; spending 2 feats to be able to use FoB with a nonstandard weapon seems enough of a cost to me. Especially since they'll be dropping from 2d8 or 2d10 base damage at higher levels to d10 maximum.
I would consider removing the BAB prerequisite, but it would only seem logical that a good amount of effort would be taken to add a weapon to the flurry of blows group. And, yes, while they will be dropping a die of possible damage from their unarmed damage, the thing is that, per the rules, you can't have gloves with damage and attack bonus enhancements (though you can have touch based damage modifiers, such as flaming gloves that deal +1d6 damage on touch).
Gauntlets are not considered unarmed attacks, therefore this hand covering doesn't work for Monks. Gloves are supposed to be Dexterity/Quickness/Touch enhancements, not weapon enhancements. People have broken this rule so that higher level monks aren't left behind, but that's not what should have happened. Instead, a feat like this should have opened up the options when a higher attack bonus is needed or when attacking something that would otherwise cause harm when using unarmed strikes (jellies, cubes, and the like).
So, the best way to give a monk a higher attack bonus is through the use of weapons. But, one of the big let downs is not only the selection of weapons, but the rarity of those weapons in modules and other pre-generated content.
I would prefer a more simpler wording to the effect.
"The selected weapon is treated as a special monk weapon in addition to the ones granted by the monk class".
I'm sure someone else can make it even more elegant.
It has to be written that way so that it doesn't become overpowered or take away from the concept of flurry of blows. The purpose is to replace only your primary weapon in the flurry of blows, not either or both. I believe if we were to allow it to go beyond that, it would move too far away from the purpose of flurry of blows and into just a monk version of open-ended two-weapon fighting.
With it just being treated as a special monk weapon, you could end up with a monk dual wielding bastard swords (Monkey Grip or a similar feat makes this possible), which really isn't in line with the concept of flurry of blows. It's worded to keep it from getting out of hand.
I agree with the others: Just state that it's a special monk weapon now, and drop the Weapon Focus and/or BAB prerequisites. we're not talking madly overpowered here, especially since we're talking about the monk.
On a related matter: I had the idea of letting monks have bonus damage to unarmed strikes or special monk weapon attacks in lieu of the flat unarmed strike damage they're getting now, so that they can keep using those weapons without them falling behind in damage. Maybe they get a separate progression for weapon damage bonuses, or make weapon attacks with a bonus equal to the unarmed strike damage of a monk 4 (or so) levels lower.