What I did in my Giana sisters STE conversion is that when I enter a new level I setup several screens with all the graphics so that the scroll routine only has to do that and not fill in any new data. I tried that at first, but it takes a lot of processor power to redraw the new 16 pixels that will come into view. If you scroll diagonal you need to draw both horizontal and vertical data so that's even more.
The problem is ofcourse that the hardware scroller can't scroll an infinite wide screen. If you need a wider screen than just 4 * 320 you just have to create a copy of the last screen and put at the beginning of a new memory area that you jump to then you walk past the first 4 screens.
For example, if I need a 7 screen wide playfield I just put them in memory like this:
1 2 3 4 | 4 5 6 7
So I have to waste one 320x200 area with a copy of the last screen in the first buffer but it's not that much memory you loose. 8 screens only take 32k*8 = 256k and it shouldn't be a problem for an STE.
ST / STFM / STE / Mega STE / Falcon / TT030 / Portfolio / 2600 / 7800 / Jaguar / 600xl / 130xe