While not used in every ground attack mission, P-38s did carry 1000lbers in level flight bombing runs. They'd be led by a P-38 Droop Snoop pathfinder equipped with a Norden bombsight and would all drop when the pathfinder released its eggs. How many missions were run like this? Probably hundreds, if not more.
ack-ack
I went through the 428t FS, 474th history just to check as they flew 38s in the ETO until the end. The flew ground attack and consistantly carried 2 x 1000 pounders for dive bombing or general ground attack from November 44 on. It was 2 500 pounders prior to this time, so they were lugging 1000 pounders almost daily from November 44 to May 45. They also carred 2 x 2000 pounders on a few occasions, again in ground attack. They didn't do much droop snoot stuff.
Quoting an e-mail from a 474th pilot who I correspond with now and then.
"We even carried two 2000 lb bombs a few times. Had to use wooden sway bars that banged up the bird on bomb release
so we didn't do it often and 1k bombs were probably just as effective. Both 2k bombs had to be dropped together-on pull out,
the bomb shackle would come off from g force."
As for the Mustangs. Even in Korea where they were closer to the front lines and didn't lug DTs, they carried a mix ord load of Rockets and bombs or napalm. It makes sense to have that kind of load out I think. There is a great book on the Mustang in the Korean war with all kinds of photos and there isn't a single photo of a 10 rocket load out, but all kinds of photos of 51s with 6 rockets and bombs or 6 rockets and napalm tanks.