You need to modify C source and recompile. Either that or get Epoch or Lucid Emacs instead. Patches have been written by Robert Forsman <thoth@reef.cis.ufl.edu> and Johan Vromans <jv@mh.nl> to allow Emacs to iconify itself and by Matt Wette <mwette@mr-ed.jpl.nasa.gov> and Manavendra K. Thakur <thakur@zerkalo.harvard.edu> (for 18.57, plus icon geometry) to allow Emacs to start up iconified. I don't know which of these patches work together.
Anonymous FTP:
/csi.jpl.nasa.gov:pub/emacs.patch1 (Matt Wette) /ftp.eu.net:gnu/emacs/FP-Xfun.Z (Johan Vromans) /ftp.urc.tue.nl:pub/tex/emacs/FP-Xfun (Johan Vromans)