I don't know what "IE hack" you're talking about here. ApiSandbox is passing $label so it can set a 'click' event on it, FieldLayout's sometimes-forwarding isn't sufficient. There doesn't seem to be a documented way to get the generated label element, only to set it.
I didn't realize that this even could be done, much less that somebody would actually be doing it. Sorry.
I think that this entire thing is actually a bug in OOjs UI – you shouldn't need to override bits of FieldLayout to make it focus the field when the label is clicked. I filed T164506 for this. I think we can clean up a few gnarly things if we make this work consistently, actually.