Those of you privileged to go to the annual Arbortext meetings might be able to answer this question: When will Arbortext Editor/PE support XML Schema 1.1? I believe the standard is in final draft stage as of 12/2009. Some vendors, such as Saxon, already support many features of the standard.
If there have been any announcements of this support, please let me know.
I have no insight on the timetable for XML Schema 1.1 support in Editor, but in the meantime, you might have a look at the Schematron support in Editor 6.0 to see if it might serve your needs.
As I recall, this feature is not as tightly integrated as the Schema support, so it would probably come in the form of a periodic check, such as pre-save or on-demand, rather than the ongoing checks performed by Editor as you work. As a stopgap, though, it may get you closer to where you want to be while you wait for Schema 1.1 support.
In the interests of maintaining Schema 1.1 validation as a goal, you may want to look at this "XSD to Schematron" tool . There may be others like it available. The idea here would be to "compile" the Schema into a set of Schematron rules that performs the same checks. I don't know how much of XML Schema this tool supports, or whether it includes any of 1.1, but it might serve as a starting point.
For your purposes, you might use a similar approach (or just a manual process) that converts to Schematron only those parts of the Schema that Editor does not yet support, such that your Schematron validation pass would assume an input document that is in compliance with the rest of the Schema and only check for violations of those additional constraints.
Schematron is very open ended and therefore provides a sort of superset of schema languages, functionality-wise. Therefore, it should be able to encode almost any constraint. The downside of that power is that it is very difficult - possibly impossible in the general case - to implement efficient incremental checking of such constraints in the fashion of Editor's support for Schema, thus the value of keeping as much as possible in Schema and only resorting to Schematron where necessary.
Sounds interesting, but we're deeply entrenched in XML Schema. The Oxygen people just announced full support of XML Schema 1.1 in their latest release. I guess I'll open a PTC case and ask the question. It used to be nice when Arbortext folks participated in this forum...