"When dealing with guns, the citizen acts at his peril", Superior Court of New Jersey, Appellate Division, Judges SHEBELL, BAIME, and BRAITHWAITE in STATE v. PELLETERI, Oct. 21, 1996.
This quote exemplifies the attitude towards firearms in NJ. The NJ firearm statutes are vague and devoid of common sense. The current attorney general guidelines on "assault weapons" are over 15 years old. You are ultimately subject to the whim of the police, prosecutor, judge, and jury. Mere possession without criminal intent can cost you 6 figures of legal fees and fines not to mention significant jail time and loosing your firearm rights in any state forever.
NJ has about 8 million people who hate guns and maybe only a 1000 gun people. So when you exercise your 2nd amendment rights in NJ, there is a high probability anyone you will interact with will be vehemently anti-gun and will treat you like a criminal. The law won't save you. Just ask Brian Aitken or the kid who was arrested for handgun possession when he had a airsoft in his car, or the guy who shot an attacking pit bull and was charged with handgun possession, or the CCW permit holder from Utah arrested for handgun possession in his declared checked bag at Newark Liberty airport or the numerous people arrested over the years for possession of "assault weapons" when their weapons were perfectly legal.
I would think thumbhole stocks would be OK as they were used to comply with pistol grip ban of the '94 Clinton AWB, but my concern is they are uncommon and you could encounter a police officer unfamiliar with guns who thinks they look evil and would arrest you first and ask questions later.
My advice has always been to have guns in NJ that look like everybody else's. I run a target crown, low profile gas block and an A2 stock. Hopefully, there should be no way they can think I have a collapsible stock, bayonet lug or flash suppressor. I would love to have a pinnned Magpul stock, PWS FSC-556 compensator and a regular FSB but to me its not worth the risk of wearing cuffs on the road side trying to explain to some well meaning police officer why I don't possess an assault weapon! My assessment of your question is any potential benefit of a thumbhole stock over an A1 or A2 stock and pistol grip doesn't justify the substantial risk.