Since I didn't receive a response I made a educated guess at accomplishing this and it seems to have turned out as hoped. Here's what I did in case someone down the line has the same question and finds this in a search.

The metal part wasn't pressed in so much as the back part essentially acted as a rivet. First I ground down the flattened area on the back side and pulled it out with vice grips, then I got a solid piece of iron to act as an anvil, and my 8 lb sledge. I then placed the ball under the arm so the small "rivet" part is sticking up through the arm, a few relatively moderate swings of the sledge and it was secure. The metal ball the slider is on has a flat part which sticks out far enough against the anvil that you won't break the plastic in this process. I did use some scrap wood to make sure the regulator arm was approximately parallel to the anvil before I secured the slider ball to it. All told it only took a few minutes per slider to replace, it took me longer to make this post than replace all 3 on the regulator in question.