Last month we started working on a web-based XML editor with just these use cases in mind as part of the ANGLES project, a Digital Humanities Start-Up grant funded by the National Endowment for the Humanities. It is still early days yet but we've made what we feel is a pretty good start. ANGLES extends the ACE editor you mention and connects it, for the moment, to a SAX-style parser to do basic validation.
As I said, development has just started. We'll be continuing to work on this project through at least Fall 2013 and would love to talk to you (and any other interested parties) about this project.