Showing and hiding elements does not create any events as far as I know, so live will not help you.
However, since you are not adding new elements or changing their order, you can set the correct tabindex from the very beginning. In any case, the browser will ignore hidden or disabled items. Run your code without filters :visible and enabled :
$(':input, a, span.ValidatorClass') .each(function(i, e) { $(e).attr('tabindex', i) });
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