If we have some string which looks like EL we should threat it as EL and provide OpenOns for it.
The list of all the available ELs is provided by PageContextFactory and it works for Java, XML, xhtml and jsp files if the project has KB builder.
Code completion and validation uses the PageContextFactory to find ELs so it works for java sources.
But OpenOns seem to work only in Java sources of Seam projects. Also it doesn't work for XMLs if EL is within some tag body.
OpenOns should use PageContextFactory to figure out is the string is a EL and work w/ any EL supported by PageContextFactory.