The brake thing is easy - flash hiders are not allowed if it violates the AWB feature count. If its certified as a "brake" and not a "flash hider" by the ATF, it is legal in NJ. It is the same criteria that applied across the country during the Federal ban. If the manufacturer or retailer doesn't clearly state it, simply ask if they obtained ATF certification. (a letter)
Odds are, any new product will NOT be certified as companies don't have to anymore...
That said, you can put any flash hider on any firearm in NJ as long as it doesn't violate the AWB feature count. For example, a bolt-action rifle can have them, a pistol can have them, M1As can have them, etc.
Edit to add: Flash hiders, per se, are NOT banned in NJ. The AWB states that you cannot have more than 2 "features" on semi-automatic firearms. One of these features is a flash hider...