I think you should still have half an hour to edit unless someone responds...
David, when stating you cannot get something to work it's important you also tell why
But when I tried your "replace (~F) with \t$1" for myself it became real obvious straight away: the code ~F does not only indicate "the footnote reference number" but also the footnote itself in the text. So, while it works in the sense that you get a tab before the footnote number code, you get loads of extra tabs in your text as a bonus.
There are several ways to circumvent this. The most obvious is to limit the replace operation to just the paragraphs styled with your footnote style. If that's a unique one (mine usually is), it'll work straight away. That's Eugene's first answer in the previous thread -- possiblly you forgot to set this Paragraph style in the Find Format field.
Another way is to only add a tab when the footnote number code is at the start of a line:
(You are correct in noticing the "lookbehind" trick cannot be reversed to a lookahead for just 'start of line followed by the footnote number code'. It's a Limitation of InDesign's particular flavor of GREP.)
This one may fail if you ever have a footnote at the start of a paragraph
Yet another way is to try to make use of the vertical position of the footnote number code. If your references are in superscript (which is fairly regular) and the numbers in the notes themselves are not (which they may or may not be), you can look for
~F (+ position: normal)
and replace with itself plus a tab before, as above. (A variant could be to use a character style for the superscripted references and look for [None] character style to pick up the ones inside the footnotes.)
Yes, I should have mentioned that I, too, use a special footnote paragraph style that I have ID apply to the notes. I tried again just now, stepping through the notes one by one, replacing ~F with \t$0. Nothing. Not only did the file look the same, ID reported the file as unchanged. But if I hit Change All the GREP replacement works!
I'm vaguely aware of differences in Change vs. Change All (I seem to recall that applying a paragraph style to whole paragraphs via Change All can zap local formatting). And certainly I've been burned by using Change All without sufficiently testing my parameters with Change. But this is a case where caution is counter-productive. Live and learn.
Thanks for your help,