Use bibliography numbers and citation from other file

Save the following in your working directory as xc.sty:

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{xc}
         [2011/09/02 v0.1 eXternal Citations (DPC+EG)]
\def\externalcitedocument{\@ifnextchar[\XC@{\XC@[]}}
\def\XC@[#1]#2{{%
  \makeatletter
  \def\XC@prefix{#1}%
  \XC@next#2.aux\relax\\}}
\def\XC@next#1\relax#2\\{%
  \edef\XC@list{#2}%
  \XC@loop{#1}}
\def\XC@aux{%
  \ifx\XC@list\@empty\else\expandafter\XC@explist\fi}

\def\XC@explist{\expandafter\XC@next\XC@list\\}
\def\XC@loop#1{\openin\@inputcheck#1\relax
  \ifeof\@inputcheck
    \PackageWarning{xc}{^^JNo file #1^^JLABELS NOT IMPORTED.^^J}%
    \expandafter\XC@aux
  \else
    \PackageInfo{xc}{IMPORTING LABELS FROM #1}%
    \expandafter\XC@read\fi}
\def\XC@read{%
  \read\@inputcheck to\XC@line
  \expandafter\XC@test\XC@line...\XC@}
\long\def\XC@test#1#2#3#4\XC@{%
  \ifx#1\bibcite
    \bibcite{\XC@prefix#2}{#3}%
  \else\ifx#1\@input
     \edef\XC@list{\XC@list#2\relax}%
  \fi\fi
  \ifeof\@inputcheck\expandafter\XC@aux
  \else\expandafter\XC@read\fi}

\endinput
%%
%% End of file `xc.sty'.

Then say, in your derived.tex document,

\externalcitedocument{main}

(where main.tex is the document you are taking the citations from). Now

\cite{xyz}

in derived.tex will do what you want.

The interface is just the same as xr's: you can also specify a prefix

\externalcitedocument[A-]{main}

and use \cite{A-xyz} in derived.tex.

The xc.sty file has been obtained from xr.sty with some very simple transformations:

1. xr -> xc
2. XR -> XC
3. \externaldocument -> \externalcitedocument
4. \newlabel -> \bibcite