Couldn't the Chinese simply place a couple of HQ-9Bs and paint the aircraft? Would that technically be an act of war?
The Chinese ADIZ extends well outside the range of land based HQ9s.
It won't be an act of war, but it will be a massive escalation and clear provocation as Thunderchief already pointed out.
What more, doing so would fatally undermine China's message that its ADIZ is not intended to restrict freedom of navigation within its declared ADIZ. Using SAMs to paint a plane is more in keeping of something you'd to to enforce a no fly zone.
On top of all that, using HQ9 in that way could compromise its effectiveness by allowing the Japanese and Americans to get a record of its wartime operating frequencies and characteristics.
What others typically do to enforce their ADIZs is to scramble fighters to intercept any military aircraft from specific countries that enters that ADIZ, get a visual and 'escort' it until it leaves.
China isn't even going that far, thus far, it seems content to just get remote ID of a foreign military aircraft that has entered its ADIZ unannounced. If that foreign aircraft looks to be only transiting or is otherwise unthreatening, the PLA will not even bother getting a visual ID on it.
My guess is that the PLA defensive escalation list goes something like this:
1) ID using long range land based radar
2) Scramble alert fighters/divert AWACS to get ID using air base radar, radio asking for ID and intensions
3) Have alert fighters close to within visual range to positively ID, radio asking about intensions and shadow
4) have alert fighters get in real close, physically signal foreign pilot and radio warning that they are about to enter Chinese airspace
5) Final warning issued just before foreign aircraft enters Chinese airspace. Aggressive maneuvers to try and get the foreign aircraft to alter course. Maybe going as far as firing tracers across the bow.
6) Issue notification that foreign aircraft has entered Chinese airspace, demand foreign aircraft change course and land at designated Chinese airbase.
7) Weapons lock and final warning to comply
8) Shoot down.
What I suspect the Japanese, and maybe the Americans as well, are doing right now is to gradually edging closer and closer to try and test out at just what point the PLA moves from one level to the next on its escalation list. They will hopefully stop before the PLA has advanced to step 5, but definitely before step 6 as that would be a stupid own goal to violate Chinese airspace with armed Chinese fighters right on your 6.
