Yep, now that I have the playfield somewhat apart I found this cool mech under the apron that switches between letting the ball drain normally or returning it to play from between the flippers, which burns off the extra balls indicated by stars on the backglass. I can give you a pic of that mech for now, pre-cleaning.
They used the mech in Set Up and Post Time as well, and after further reading it seems Roto's Add-A-Ball works more like those games' Extended Play feature. It doesn't count up more actual balls, it awards stars like on Italian export games, and since I haven't gotten it working yet I can only presume the free balls get launched back from between the flippers immediately, so no pause between "balls" until you burn off your stars and your turn is over. Sounds kinda fun, actually, and from the looks of things there are a lot of chances to win these stars, the game is all about getting the big spinning arrow in the backbox to spin and win stars.
Don't hold your breath waiting for more, I'm a 99 projects at once kind of guy, but maybe posting this will give me some gumption for this particular project... I am eager to play it!