The camera keeps metering anyway, whether you use a function like this or not. So use AE lock.
I'm sorry about my usage of the word "metering". I meant influential metering, as in influential on the Av, Tv, or ISO values. Passive metering as feedback is fine for what we were talking about, and it can remind us that the light has changed.
I haven't tried AE lock since maybe my 10D, but I assumed that it times out after releasing the shutter button, and I just tested it on the R7, and it is the same way. What we're looking for is something that just "stays" through metering periods and sleep/wake and on/off cycles without a lot of fuss and using up back buttons or keeping the shutter half-pressed. IOW, until a change is made by the user due to changing light, the same pairings of Av and Tv value are held constant.
The custom function you mention, I haven't seen it make any change with a single prime lens, varying the Av value in either Av mode or M. I'll have to look into it more, but it seems to have no effect with a single prime lens. Maybe with a variable-open-f-ratio zoom it will have an effect when you zoom in (dropping the shutter speed), or when changing lenses and the Av value that you used previously is not available after the change. IOW, you were using a 400/5.6 at 5.6, and then put on a 1.4x TC, the ISO would double or the shutter speed would halve. If the camera was already set to f/8 before you added the TC, then nothing would happen. That's how I understand the custom function at this point.
I'd have more enthusiasm for experimentation to figure these things out, but the R7, unlike the R5, can't save all your setting to a file, so you have to remember to change everything back again after the experiment, or get burned the next time a fleeting photo-op presents itself. I don't know why Canon has to be so stingy with this very useful feature. Yes, you can play around in a C# mode, but some of these settings are global, and stick when you return to your regular modes. The menu system really should have symbols on the entries that tell you when settings are global, and when they are mode-exclusive.
