The HALPRO raid was almost completely ineffective (more bombs landed in the Black Sea than at Ploiesti). The Allies did not bomb Ploiesti again until TIDAL WAVE on 1 August 1943.First thing, we're talking about 1942, not 1943; it was mid-1942 when a small US bomber force attacked Ploesti the first time.
So - if there was similar operation ITTL, Germany would push Bulgaria to protest, which they probably would anyway. Would Germany really force Bulgaria to declare war because of the pinprick of HALPRO? Would the Allies risk forcing Bulgaria into the Axis in order to repeat the HALPRO fiasco?
In both cases, I doubt it.
The Allies would have no right to complain about Bulgarian planes defending their own country's airspace. They would be clearly and unmistakably in the wrong by all the laws of war.Or the Bulgarians could indeed try and tackle the Allied bombers - which would bring the Bulgarians on the Allies' black list anyway.
Maybe they would take this course. But it would not be until they had the strength to inflict a major blow - which means mid-1943 at the earliest. I'll further note that TIDAL WAVE was flown by day, beyond fighter range; while it inflicted substantial damage, the losses were horrific. The Allies did not attack Ploiesti again tlll 5 April 1944.
Given all the difficulties of attacking Ploiesti, the limited effects of the attacks, and the diplomatic headaches entailed, I find it implausible that the Allies would commit to a sustained air campaign against Ploiesti while Bulgaria, Greece, and Yugoslavia are neutral.