Unfortunately, I can only compile Universal for OS X 10.4 and up. I may try to compile an 10.3 version, but rght now it looks like it will probably require at least v10.4.
There is a way to compile it so that the PPC side uses 10.3 libraries, and the Intel side uses 10.4 libraries. However, 10.3 is very old and almost anyone (actually,
everyone if you use XPostFacto) who is still using 10.3 can upgrade to 10.4.
The only reason to support 10.4 even, is because 10.5 cuts out G3 support. Some people may still be using iBook G3s and such. Anyone with a G4 can install Leopard (if their machine is slower than 867 MHz, a simple Open Firmware hack will take care of the Leopard Installer's requirement)