OT, but related to an ETP discussion about a year ago lost in the depths of the boards (or was it just in my world?). Anyway, this suggestion is not for 5.0, but maybe 5.x or 6.0?
What if the question is rephrased to:
What editors can OpenACS import/export pages to/from?
Perhaps ETP (in the context of CMS) could function by allowing the user to download the page (in the format of their favorite editor), make changes, and then upload the file. Or, just upload a file to create a new page etc. etc.
Openoffice has an expanding set filters for importing/exporting file standards for various editors. Maybe these filters could be a jumping point for developing this "knowledge-base" system. It could be expanded beyond text formatting for applications involving graphics, such as shared development of engineering drawings, or other formats etc.