Coding a NES emulator

[oRBIT]

I've been coding a NES emulator since the 90's. Well, not all the time but from time to time.
One thing that have bothered me all these years are how to solve proper graphics emulation. There are a few tricky parts to solve on low-end Amigas.

The NES have 4 screens ordered something like this:
2 3
0 1

There are usually only memory for two of them so the other two are mirrors. Games usually have horizontal or vertical mirroring. Horizontal means, 0-1 are the same screen and 2-3 are the same. Vertical means 0-2 and 1-3 are the same.
Emulating this on Amiga requires 4 screens, or preferable even 6(!) screens.
Assuming you're playing a game like "Super Mario Bros" that scrolls from left to right. Screen 0 is used first, flips over to screen 1 that keeps scrolling and actually shows screen 0 again. Hope you understand what I mean.
This is the problem.
I don't want to keep updating 6 screens in chip-RAM for this.

Could there be a smarter solution? I'm interested in ideas for AGA-setups. I guess in theory you could use the blitter but blitting the entire screen in 50fps, well, that wouldn't be fast.