Apparently, maybe during the transition to the wiki, we lost a proposal made by David Wheeler to allow the <table:named-expressions> element also in the <table:table> element to be able to store sheet-local named ranges and expressions.
http://lists.oasis-open.org/archives/office/200608/msg00001.html
See also http://lists.oasis-open.org/archives/office/200611/msg00085.html
The current OpenFormula draft in section "5.11 Named Expressions" defines a syntax that provides sheet-local named expressions.