No, it’s not OK.
Appendix F of the XML spec says:
Because each XML entity not
accompanied by external encoding
information and not in UTF-8 or UTF-16
encoding must begin with an XML
encoding declaration, in which the
first characters must be ‘< ?xml’, any
conforming processor can detect, after
two to four octets of input, which of
the following cases apply.
Ah, but, section F is non-normative, you say.
Well, section 2.1 gives the production for a well-formed XML document, thus:
[1] document ::= prolog element Misc*
…and in section 2.8 we get the production for “prolog”:
[22] prolog ::= XMLDecl? Misc* (doctypedecl Misc*)?
[23] XMLDecl ::= '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'
So, you can omit the < ?xml declaration, but you can’t prefix it with anything.
(Incidentally, “Misc” is the category that comments fall into).