Is there a way to 'reference plain text' like with ‘reference Inline Footnotes’?

I can only imagine two ways that you might run into trouble using that code:

  1. If the string you are using to flag endnotes (‘ZZZ’ in my code) has anything that the tex processor might treat as a special character/code, that might cause you grief.

  2. If not that, it looks to me like the only place for a hitch would be in the url{} function and the IfBeginWith{} function not playing nice together.

(I think it unlikely to be the fnote cmd call, because this literally invokes the original footnote function and nothing more. The let cmd is very low level TeX. So, nothing should be hinky with that part.)

You could verify that by trying something like:
\let\fnote\footnote
\renewcommand{\footnote}[1]{\fnote{#1}}
Which should just return footnotes unmolested.