I think MAME documents where the zoom registers are even if it doesn't hook them up. I did a bit of research into them last year actually, although never submitted the code.
The slightly concerning thing, from the videos I saw, is that it almost looks like the zoom thing is playing with the CRTC registers, and increasing / decreasing the horizontal resolution of the screen to zoom (there didn't seem to be horizontal detail loss when zooming out from what the person testing the hardware was telling me)
also Guardians appears to set up an odd resolution with '.5' pixel sizes in order to not cut off some of the screen details.
you could hack around it and just treat it as a regular zoom, and lose detail, but in reality the hardware might be doing something a bit more fancy.
for the linescroll, I'm not sure, could be a raster effect, I couldn't see a line table.