There exists a family of architectural styles and approaches using attribute based selection, which are often used in connection with event based systems (e.g. middleware). In such system, one can define types of events that trigger arbitrary actions for observed (managed) objects, where attributes are used as a selection mechanism of a monitor (quite similar to an access control monitor). Such approaches are frequently discussed in the area of publish/subscribe systems and event-based middleware (see e.g. http://www.computer.org/portal/site/dsonline/menuitem.9ed3d9924aeb0dcd82ccc6716bbe36ec/index.jsp?&pName=dso_level1&path=dsonline/topics/middleware/em&file=EM.xml&xsl=article.xslhttp://cocoon.ifs.tuwien.ac.at/pub/debs/debs2007.pdfhttp://portal.acm.org/citation.cfm?id=966636
xowiki policies make only one step in this direction, one could certainly bind some actions to the identified situations. However, this is a limited form (maybe still useful), where the publishing part (one should be able to publish event types, for which users could subscribe) and the user subscription part are missing. one could probably use the notification part of OpenACS.