Not that I had much expeience in HW-banging, but .....
What you describe in the 3rd paragraph was pretty much what came to my mind when I read your 1st question in the 2nd paragraph :-D
And yes, getting that right would be nightmare,l but once you have figured it it would be easy.
The scroll-texts were offcourse vektorized fonts (most I remeber only consisted out of a few points), all you had to do was move these points based on a static sine-table.
Remember that resolution was rather low, so having one value for every 5° would still look smooth. Add some obscure shifting and bit-rolling to replace those slow divs/muls and your set.
