I'm not sure how Tim is using "support" in this case.
In the XML Rec, production 12 [1] is for PubidLiteral which is described
in the XML Rec [2] as follows:
In addition to a system identifier, an external identifier may
include a public identifier.
All FPIs are public identifiers (the set of FPIs is a subset of the
set of public identifiers).
Finally, whether or not you consider this "support" of FPIs, the original
question is not unreasonable; the W3C could still opt to define an FPI for
the XML Rec so that people could write a notation declaration such as:
<!NOTATION xml PUBLIC "-//W3C//NOTATION XML 1.0//EN">
(Note that XML notation declarations [3] do not require a system identifier
as external identifiers for entity declarations do.)
I am not aware of this issue being raised before, so I believe the current
answer to the question is "no, there is no official FPI for XML," but I see
no technical reason there couldn't be.
[1] http://www.w3.org/TR/REC-xml.html#NT-PubidLiteral
[2] http://www.w3.org/TR/REC-xml.html#sec-external-ent
[3] http://www.w3.org/TR/REC-xml.html#dt-notation