;

I don't think it can be done.  Have you tried it through a computer, I don't know if that gives you more control.   Also see if magic lantern will do it.

Through computer not really feasible for this specific, though that would mean I could manually set it remotely, which would help. Actually, I think I've read somewhere that you can set up a phone to change camera settings, which I may have to resort to.
What's Magic Lantern? Googled it, and I don't see that it offers this.

maybe using Magic Lantern there's an ISO fix ?

Tell us what the project is and maybe someone can figure out a solution.

Small birds (eg. finches and tits) in flight.

I'm not looking through the viewfinder, as the technique involves watching for the birds to leave a provided 'perch', (if I can see the bird in the viewfinder/live view, I can't click in time) and I often miss that the light has changed [1] because I've got whatever photochromatic lenses are called nowadays.

I know one option is to get the birds to fly through a beam and photograph themselves, using flash - but flash isn't allowed in the nature reserve where I'm shooting, and my garden isn't big enough to get a nice out-of-focus area anywhere to shoot against.

[1] Which is why I need the auto ISO.

Do these directions help? It's been a long, long time since I've had to do anything similar. I spent a lot of years shooting products in a studio setting. They never moved.

http://www.lanephotoworkshops.com/photographing-birds-in-flight/

Yeah, tx; but Auto ISO is needed there, and that's what I can't seem to do.
He's talking about big birds (he mentions Great Blue Heron flying overhead). These fly much more slowly, so he is discussing AF techniques, which don't work for small birds (Maybe the 5D3 is better, I can't say).
As far as I know there are only two techniques for small, fast birds, either the shoot themselves through a beam or the way I'm trying, which is prefocus on a feeder, set manual focus and recompose so that the feeder is 'just' out of the image. Provide a perch on a plane with the feeder, and start shooting on continuous as soon as the bird leaves the perch. 1 success in 100 would be a very good success rate  - there's usually only even a bit of bird in about 1 in 10 shots (the little darling's don't always fly in a straight line, or they hop to the ground to pick up dropped seed, or something spooks them and they swerve away in mid flight, or they turn their head away, or they cover their eye with their wings:

(You wouldn't believe how often that happens!)
I'm making some progress with this, but the auto ISO would definitely help.
I think my 40D has Auto ISO, but even 'not for stock', I don't like ISO over 400 from the 40D - if it's not direct sun (too contrasty) I'm in the ballpark of 1600 ISO, often higher; 800 if I'm lucky.
At the moment, I'm shooting on AV, but the light changes a lot, even when it just looks 'cloudy bright' to me. When it's sunny, the ISO goes down, yaaay; but the contrast is too harsh.

Shady - you have much more patience then I do!

Try it in the View Mode in Stills/Movie setting with camera on Tv. It sort of works with a limited selection of speeds. If you are lucky it may fit your needs.

Tx all.
A few things to try out; throwing money at it, not an option.
Patience for this I have tons of. I got my first camera to photograph birds and it's still my first love tying with other creatures.