I'm not an expert but I've got a lot of PowerPC eMacs. I am going to have to disagree on this one. I just finished installing OS X Tiger on this machine via an external DVD drive. I selected that drive by holding the Option key when booting and selecting the drive.
Works great. I've never been able to install Leopard using a DL disk via the built in drives. Now, granted these are old eMacs. Maybe they are dirty lasers but my external drive does a great job of it.