No i haven't, at least not on a PowerMac G5. I did do this however on an older PowerMac G3 MT a long time ago. It worked but didn't have that great of a signal, or maybe range?
If you are considering buying one, i would first look into finding an Airport Express card, on LEMswap or maybe even here. I think the G5 will have a better antenna, since it wraps around the case on the inside. I think you should be able to find one cheap enough, i just bought one not long ago for $25.