Re: “Almost all strokes in Chinese calligraphy go from top to bottom, so if someone wants to use a brush stroke as illustrative of a diagonal line, there isn't much besides pie to use.”
Well, tiao3 comes to mind, which ramps upwardly, left to right. Also, as far as representing a diagonal line, the na4 stroke is a right-downward diagonal.
As far as throwing in a “flat trajectory,” I’m not sure that’s stricktly possible (a throw necessarily follows an arc), and I might translate that Xinhua Zidian entry as “throw forward smoothly.”
I read the same entry in the Jingxuan Taijiquan Cidian, and as you point out, it clearly implies that the pie-ing of the body preceeds the deployment of the fist. Rethinking this, another plausible explanation deserves consideration. Pie in the third tone, in the compound pie3zui3, means to purse one’s lips, curl the lips, or to scrunch up the mouth to express contempt or doubt, etc. So by this interpretation, pie could refer to the gathering up of the body—arms folding and closing toward one’s center—prior to the back-fist strike. It would fit the grammatical construction of the name Pie Shen Chui. But then again, Yang Chengfu’s form description clearly used pie as a verb for the action of the fist.
There doesn’t seem to be an easy answer to this!