No, there's no easy way to do this. The problem is that the built-in form block creates all of the field markup in the controller, not the view (which is a bad architectural design, but one that nobody has the motivation to go and clean up now). So the only way to modify that stuff sanely is to do string manipulation in the view. If you look through the code of this addon's view.php file, you'll see it's already doing that in some places. So theoretically you should be able to extend this technique to work for tab indexes as well.

BUT, that being said... I'm not sure how you'd determine what the tab index should even be for each field -- since all the fields are outputted in a loop, you'll have no way of knowing in the code which field should be for which tab index (unless you just wanted them in numerical order... but if that's the case then why do you need them at all, since that's the default browser behaviour anyway?)

Thanks for your speedy reply.

I suspected it wouldn't be easy. I'll think I'll stick with the default browser behaviour :)