I think this is a case of being in violent agreement -- see some earlier replies in this thread -- 
Pragmatically, if you are going to hew to marc-8 encoding transported in XML -- you are losing the usefulness of standard tools for xml --

-----Original Message-----
From: Code for Libraries [mailto:[log in to unmask]] On Behalf Of LeVan,Ralph
Sent: Tuesday, April 17, 2012 4:21 PM
To: [log in to unmask]
Subject: Re: [CODE4LIB] MarcXML and char encodings

>> No -- it is perfectly legal - -but you MUST declare the encoding to
BE Marc8 in the XML prolog,

> Wait, how canyou declare a Marc8 encoding in an XML 
> decleration/prolog/whatever it's called?

Nope, you can't do that.  There is no approved name for the MARC-8
encoding.  As Andy said, the closest you could get would be to make up
an experimental name, like x-marc-8, but no tool in the world would
recognize that.