EL5 ppc is well supported, but far fewer people use it as a desktop thus it is not likely to be a better platform for PPC clients. Fedora's kernel and/or yaboot needs fixes for netboot apparently.You might want to look at the EL5-based K12LTSP distribution since it embodies a lot of the work required to boot PPC clients from the quite capable and proven K12LTSPel5.
I've done it with both EL4 and EL5. Matter of fact, I've had x86, PPC, and even UltraSPARC boxes running as thin clients...from the same K12LTSP server...simultaneously. :-)
Basically, I had to build a custom /etc/dhcpd.conf for it and copy /opt/ltsp/ppc and /opt/ltsp/sparc over. That, and make an /etc/hosts file that would accommodate it. But it really was not very hard.