A while back I did this. Here is a rough outline od what I had to do:
1) First you need the binutils for the platform you want to compile to. Download odcctools from this site (use the cvs version) and follow their instructions for installing.
http://www.opendarwin.org/projects/odcctools/2) you need to create a fake root dir like: $HOME/darwinroot
copy at least the /System and /Frameworks and /usr directories (you may have to copy more than this) from your Apple or Darwin computer to $HOME/darwinroot
3) now that you have these files make a folder in $HOME/darwinroot called cross. where ever you installed the odcctools you need to make links for the cross tools to be more fpc friendly. there are a bunch of files from odcc tools called powerpc-apple-darwin-* you need to make links (or rename them) so powerpc-apple-darwin-ld becomes powerpc-darwin-ld, do the same for *-ar and *-as.
4) now you are ready to crosscompile fpc. basically you need to have the fpc source and have a terminal open there.
type: $PATH=$PATH:$HOME/darwinroot/cross (or whereever you made the symlinks)
type (iirc) make all TARGET_OS=darwin TARGET_CPU=powerpc OPT="-Fl/$HOME/darwinroot/usr/lib"
if that succeded you can install it to whereever you want with: make install TARGET_OS=darwin TARGET_CPU=powerpc PREFIX=/cross/fpc
now copy the file ./compiler/ppccross somewhere you will be able to find it as it's the compiler you'll need to build powerpc programs
5) configure your /etc/fpc.cfg file.
add a section like this:
#IFDEF powerpc
-Fu/cross/fpc/lib/fpc/$fpcversion/units/$fpctarget/
-Fu/cross/fpc/lib/fpc/$fpcversion/units/$fpctarget/rtl
-Fu/cross/fpc/lib/fpc/$fpcversion/units/$fpctarget/*
-k-systemroot $HOME/darwin/cross
#ENDIF
whenever you want to crosscompile you have to have ppccross and the symlinks to powerpc-darwin-* in the PATH
and you should be able to just do ppccross someprogie.pas and it will create a darwin executable.
I may have mistated some things (or most everything) as it's been a while since I did this.
Andyman