Clickable Table of Contents without using Section/Subsection/etc. Tag

You could use one of the packages mentioned in the FAQ answer https://www.texfaq.org/FAQ-secthead. But it is usually easy to redefine sectioning with the built-in @startsection.

usepackage{hyperref}
setcounter{secnumdepth}{0}
makeatletter
renewcommandsection{@startsection
  {section}{1}{0pt}{-12pt plus -1fill}{12pt plus 1fill}%
  {hugescshapebfseriescentering}}
makeatother

I forget what documentation explains @startsection -- clsguide didn't have it, but you could find it in ltsect.dtx.

Edit, based on comments: The first vfill should not be discarded at a page break, and the regular section command should be retained for other types of sections, like the ToC heading. So using a random name instead of "section"

usepackage{hyperref}
setcounter{secnumdepth}{0}
makeatletter
newcommandblob{@startsection
  {section}{1}{0pt}{-1pt}{12pt plus 1fill}%
  {hugescshapebfseriescentering mbox{}*[fill]}}
makeatother

I retained the name "section" so the ToC entries are formatted the same was as regular section entries.

[I am still uneasy about the huge flexibility of vfill and wonder if the part command is closer to what appearance is desired.]

If section or some other command using @startsection is out of the question, then make sure your code contains phantomsection followed by addcontentsline{toc}{section}{<the title>}. In the original question, it would be

begin{center}
  phantom{.}
  vfill
  phantomsectionaddcontentsline{toc}{section}{Foo}%
  textsc{textbf{huge Foo}}
  vfill
end{center}

You will have to use a font family that provides bold small-caps.