It was actually towards the "end" of Poser 3 (shortly before the release of Poser 4) when the the exploit that allows MAT poses was discovered.

Source: I discovered said exploit :) I was getting tired of click-click-clicking just to set up materials for a figure, since there was, at that time, absolutely no way to save a figure's material setup other than to save a CR2. This also meant saving morph/pose data, which would quickly fill an HDD (especially easy considering HDD sizes at the time). So I figured, what the heck, let's see what happens if I copy the material data from a CR2 and paste it into a PZ2. The rest is history.

 

I also firmly condemn the practice of placing material files in the Pose Library - this is 2014.