Why can't you enumerate the values? That seems to be what you are
looking for ("restricting a type to a specific set of
string values"). Otherwise I only see regular expression patterns as a
way, but that is more complicated than enumerating the values.
I am trying to make a SOAP interface such that the client can
dynamically reconfigure its graphical user interface with no
programming changes and no recompile required whenever the web
service is updated with new capabilities. This might not be possible
with pure SOAP, and may require the SOAP message to have an embedded
XML payload.