makeatletter
defbeamer@endinputifotherversion#1{}
defbeamer@sectionintoc#1#2#3#4#5{{huge #2 hfill page #3 par}}
defbeamer@subsectionintoc#1#2#3#4#5#6{{normalsize #3 hfill page #4 par}}
defbeamer@subsubsectionintoc#1#2#3#4#5#6#7{{tiny #4 hfill page #5 par}}
makeatother

$ cat beamer-test.toc     #为了让下方内容正确高亮，此处加一个 $ 符号，下同
beamer@sectionintoc {1}{what}{2}{0}{1}
beamer@subsectionintoc {1}{1}{why}{3}{0}{1}
beamer@subsubsectionintoc {1}{1}{1}{blah}{4}{0}{1}
beamer@sectionintoc {2}{what2}{5}{0}{2}

$ latexdef -c beamer -s beamer@sectionintoc   #$
% beamerbasetoc.sty, line 108:
defbeamer@sectionintoc#1#2#3#4#5{%ifnumc@tocdepth>0%ifnum#4=beamer@showpartnumber%{beamer@saveanother%gdefbeamer@todo{}%beamer@slideinframe=#1relax%expandafteronlybeamer@tocsections{gdefbeamer@todo{%beamer@tempcount=#5relax%advancebeamer@tempcount bybeamer@sectionadjust%ifnumbeamer@tempcount>0edefinserttocsectionnumber{thebeamer@tempcount}%elsedefinserttocsectionnumber{}%fi%definserttocsection{hyperlink{Navigation#3}{#2}}%beamer@tocifnothide{ifnumc@section=#1beamer@toc@cselsebeamer@toc@osfi}%{%ifbeamer@pausesectionspausefi%ifxbeamer@toc@oossbeamer@hidetextvskip1.5emelsevfillfi{%hbox{vbox{%defbeamer@breakhere{}%beamer@tocact{ifnumc@section=#1beamer@toc@cselsebeamer@toc@osfi}{section in toc}}}%par%}%}%}%}%beamer@restoreanother%}beamer@todo%fifi%
}

definserttocsection{hyperlink{Navigation#3}{#2}}%

% usepackage{xpatch}makeatletter
xpatchcmd{beamer@sectionintoc}{hyperlink{Navigation#3}{#2}}{hyperlink{Navigation#3}{#2hfill#3}}{}{ddt}xpatchcmd{beamer@subsectionintoc}{hyperlink{Navigation#4}{#3}}{hyperlink{Navigation#4}{#3hfill#4}}{}{ddt}xpatchcmd{beamer@subsubsectionintoc}{hyperlink{Navigation#5}{#4}}{hyperlink{Navigation#5}{#4hfill#5}}{}{ddt}
makeatother

longdefbeamer@section[#1]#2{%
... ...
% before
% addtocontents{toc}{protectbeamer@sectionintoc{thec@section}{#2}{thec@page}{thec@part}%
% after
addtocontents{toc}{protectbeamer@sectionintoc{thec@section}{#2hfillthec@page}{thec@page}{thec@part}%
... ...
}

beamer@sectionintoc {1}{whathfill2}{2}{0}{1}

makeatletter
xpatchcmd{beamer@section}{{#2}{thec@page}}{{#2hfillthec@page}{thec@page}}{}{ddt}xpatchcmd{beamer@subsection}{{#2}{thec@page}}{{#2hfillthec@page}{thec@page}}{}{ddt}xpatchcmd{beamer@subsubsection}{{#2}{thec@page}}{{#2hfillthec@page}{thec@page}}{}{ddt}
makeatother

documentclass{beamer}
usepackage{xpatch}% 重定义
% ... ...begin{document}
begin{frame}tableofcontents
end{frame}section{what}
begin{frame} content end{frame}subsection{why}
begin{frame} content end{frame}subsubsection{blah}
begin{frame} content end{frame}section{what2}
begin{frame} content end{frame}
end{document}