I will be pedantic again. There may be a general expectation
among the XML congnoscenti, but there is no general
expectation "in XML", or in the wider community who assume
that XML is what the published standard says it is, nothing
more and nothing less.
Because the XML standard says so little about what a "Public
Identifier" is (and in particular, doesn't constrain it to
be either public or an identifier), we can expect to find
different sub-communities using them in all kinds of
different ways.
Certainly XML doesn't contain any rule to stop me using the
characters "IANA" in my Public Identifier, and if the rule
is written somewhere else, then I'm afraid I haven't read it
and don't feel bound by it.
Mike Kay