*Unregistered* ECC RAM might work in a G3/G4. Apple systems in general are "picky" about DIMMs (I've had really bad luck trying to use apparently perfectly good RAM pulled from Pentium IIIs and Athlon machines in my B&W G3) but the only difference between unregistered ECC and non-ECC DIMMs is the ECC DIMMs include an extra 8 data bits which the host system is free not to use. So if the system is otherwise OK with the specs and configuration EEPROM data of an ECC stick there's no reason a Mac won't work with it.
"Registered" RAM on the other hand actually has a buffer sitting on some of the control lines, and since its presence induces additional latency (while improving reliability) the memory controller actively needs to compensate for it. So far as I know no Macs which uses SDRAM supports it. I could be wrong, but I'm fairly sure that's the case.