labeling underlined text over multiple lines

I've just released soulpos, which allows this with a different approach based on savepos (pdftex or compatible required). Two passes are required:

\documentclass[two column]{article}
\usepackage{amsmath,soul}
\usepackage{soulpos}

\ulposdef{\ulnumaux}{%
   $\underset{\saveulnum}{\rule[-.7ex]{\ulwidth}{.4pt}}$}

\newcommand{\ulnum}[2]{%
  \def\saveulnum{#1}%
  \ulnumaux{#2}}

\begin{document} 

\ulnum{4}{This is short text}

\ulnum{6}{I want something to look like this text does, but I
don't want to manually break lines.} More. \ulnum{7}{And more text here.}

\end{document}

Far from perfect, but it's a start. This version doesn't work if start and end are on same line, and doesn't work if a page break happens between them. Both would be fixable.

I looked at this the other day but couldn't remember the trick to measure the position of a point along the line, then by coincidence @FrankMittelbach popped up and gave a full account of it here

How do you change indentation in the middle of a paragraph?

So here's that same code, re-arranged a bit to produce the image above (plain TeX in this version)

\hsize=.5\hsize
\newdimen\lastlinewidth

\baselineskip1.5\baselineskip

\def\ustart{%
  \ustartend{%
\hbox to \hsize{\hskip\lastlinewidth
\leaders\hrule height 0pt depth 1pt\hskip 0pt plus 2fill
\kern-\hsize
\hskip\lastlinewidth\hfill\smash{\raise-7pt\hbox{\tempn}}\hfill
}}}

\def\uend{%
  \ustartend{%
\count0=\prevgraf
\unskip\hbox to \hsize{%
\smash{\raise-\baselineskip\vbox{%
\loop
\ifnum\count0>2
\advance\count0 -1
\hbox to \hsize{%
\leaders\hrule height 0pt depth 1pt\hskip 0pt plus 2fill
\kern-\hsize
\hfill\smash{\raise-7pt\hbox{\tempn}}\hfill
}%
\repeat
\hbox to \lastlinewidth{%
\leaders\hrule height 0pt depth 1pt\hskip 0pt plus 2fill
\kern-\lastlinewidth
\hfill\smash{\raise-7pt\hbox{\tempn}}\hfill
}\hfill}}}}}

\def\ustartend#1#2{%
\def\tempn{#2}%
\leavevmode\hbox{}%
\begingroup
  \predisplaypenalty=10000
  \postdisplaypenalty=10000
  \abovedisplayskip=-\baselineskip
  \abovedisplayshortskip=-\baselineskip
  \belowdisplayskip=0pt %-\baselineskip
  \belowdisplayshortskip=0pt %-\baselineskip
  $$
\global\lastlinewidth\predisplaysize
\global\advance\lastlinewidth by -2em
#1%
$$%  
\endgroup
\vskip-\baselineskip
\vskip-\parskip     
\noindent\kern\lastlinewidth
}



\def\n{One two thr\-ee four five six seven eight nine ten. }
\def\c{Red blue yellow green. }

\n\c\ustart{3}start underline \n\n
\n\n\n\n
\n\n end underline\uend{3} first text after end
\n\n

\n\n\n\n\c\ustart{4}start again\c\n\n\c\c\n
\n\n\n end underline\uend{4} first text after end
\n\c\c\n\n\c\c\n
\n\n\n\n\c\c\n\n\c\c\n

\bye