When I specified non-metal I was merely referring to the fact that on a metal airframe you can just install an external antenna and rely on the structure itself being a ground plane. There would be no problem getting a 406 installed on our L23 for example.

I'm not arguing whether the installation of an ELT was worthwhile or not as I believe it is, I just wanted to find out if anyone had any luck finding data to refer to when making out the maintenance release in the logbook for an installation. If so, it would make things a lot easier.

I live in Canada so I'm up against a different agency than you are, though a lot of FAA things are considered acceptable here (the good old AC-43.13 for example). So far as I've been able to determine from Transport Canada, even though the ELT isn't mandatory the installation, if performed on a standard category aircraft has to be done in accordance with the manufacturer's instructions, aircraft manufacturer's instructions, AC-43.13 or approved by a DER and subsequently maintained in accordance with the regulations regarding the ELT (regular testing, battery replacement etc.)