Readers may be interested that there is an implemented language designed to do the kind of "projection" that David mentions: it is part of James Clark's DSDL implemation Jing, is
a Japanese Industrial Standard, and is currently in late draft as part 4 of ISO 91575 (DSDL).
"Namespace-based Validation Dispatching Language" (NVDL) provides a little language for routing different elements and attribute according to namespace, and sending them to different subsequent validators.
It is schema-language neutral: the validators can be in XML Schemas, RELAX NG, DTDs, Schematron or whatever. It is pretty powerful: Murata Makoto developed it first several years ago as "RELAX Namespaces" to allow one-schema-per-namespace systems.
But the problems it solves (i.e. that sometimes the best way to validate a structure is not to complicate the validation language or schema, but to have a simple schema on a simplified view of the data) are certainly as applicable to W3C XML Schemas as they are to ISO RELAX NG.
For a draft of the NVRL spec, see
http://www.jtc1sc34.org/repository/0525c.htm
For Martin Bryan's slides on NVRL and DSDL see
http://66.102.7.104/search?q=cache:CTUAKdM1YbEJ:www.xmluk.org/slides/RAL_2004/RAL-2004-Bryan.pdf+DSDL+NVDL&hl=en
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