Here in SC a vehicle can be stopped at a Safety Check i.e. "Road Block" so long as it is consistent. That is every third vehicle or every other or every one for that matter. One can just not discriminate. So pulling just motorcycles wouldn't pass.
In NY it is legal to "selectively enforce" any portion of the VTL on a given selective enforcement detail (SLE). There are parameters to the stops and as you said, there must be a pattern. Unfortunately some departments have interpreted this to mean that if they want to enforce helmet laws, they may indeed stop strictly vehicles requiring helmets. To my knowledge it has not been legally challenged yet. The whole selective enforcement detail i.e. safety check roadblock concept seems to exist in a constitutionally "gray" area...
In defense of the current interpretation of this practice there is at times a need for police departments to operate in this gray area. Without it, many fleeing criminals would not be apprehended. This can be argued both ways for ever, but in the post 9/11 era, selective enforcement is probably gonna stick around for at least a little while longer.