Still doubtful about an unlimited scheme. The black plates cost money in form of taxes paid. Nobody would want those if you could cross unlimited. China loves getting taxes.
Short term cars don't get traffic tickets because the paper plates aren't of the standard form and can't be fined electronically. If they open up the quota it's gonna be a shit show with people travelling to Guangdong to race their cars. It's already happening with the current quota scheme, albeit on a much smaller scale.
And ultimately, cars registered in HK don't pay import taxes in China. If you could cross unlimited then rich Mainlanders would use the scheme to buy luxury cars cheaper and drive them in China, same as they now do with Macau plates.