Will Robertson wrote:

> Well, I'll leave it there for now but I wonder if that implies we need a
> macro that has "setup" parameters:

Somethine like:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn
  \cs_new:Npn \tlist_rescan:nn #1#2 {
    \group_begin:
      \etex_everyeof:D { \exp_not:N }
      #1
      \toks_set:Nx \l_tmpa_toks { \etex_scantokens:D {#2} }
    \exp_args:NV \group_end: \l_tmpa_toks
  }

  \tlist_rescan:nn { \char_make_other:N \^ \char_make_other:N \_ }
    {1_2^3}
\ExplSyntaxOff
\end{document}

perhaps?
-- 
Joseph Wright