I reckon the basic laws of physics apply here.
If you apply brakes before the nose is down, it will generate a nose down force commensurate with the achieved braking.
If you do it before either spoilers come out or the AOA is lowered, you'll most likely get some mighty flat spots on the tyres, not a lot of force and a smooth nose touchdown.
With the weight on the tyres, you'll more than likely get a good smack.
With this in mind, aircraft with spoilers and the like tend to have systems fitted to prevent early brake applications often linked to nose gear WOW switching and/or main gear wheel speed.
Thrust reverse or prop discing before nose gear touchdown is pretty common though, but you'd better be sure of symetrical application beforehand, mind, hence usual SOP not to do it.
Hope that helps.