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OK I can see now I probably wasn't clear enough - I'm looking for an instantaneous, one-button way of quickly switching back and forth between very different apertures (such as wide open and f8) while having my exposure compensated automatically, preferably via ISO.
Just like when using the DOF button in photo mode live view.
And all of this while shooting video...

And my question is:
Is this physically possible with the 700D, if someone took the initiative to code it for ML?

Ah, so you want something like this: In M mode, you specify your correct exposure, and then while recording, you want to switch both aperture and iso simultaneously, in such a way they cancel each other and keep the exposure the same (without visual flickers).

If you keep ISO on auto, you can kinda do it, but it takes a while for the exposure to readjust. You might be able to fix it in post, but it might be very time consuming and you might encounter artifacts/clipping in the shadows and highlights.

I think it could easily be developed, if magic lantern can change those while recording, both at the same time to avoid flickers.
The main issue I think of is the lack of that many iso and aperture settings:

For instance (keeping constant shutter speed):
ISO 100 200 400 800 1600 3200
F/    1.4  2     2.8  4     5.6    8

So if you wanted a "slower fade" from one position to the other, you would end up seeing the 6 aperture settings, or a flash. More settings can probably be added by using different shutter speed each frame, but it might look weird in a video (if all of the speeds are high, maybe it can be fixed in post adding a fake motion blur)

With a manual lens with a clickless aperture ring, I think it can be done if you keep ISO auto, shoot in raw and then do some exposure compensation on each frame to remove (IIRC There are some Avisynth plugins to do auto exposure)