We use Perforce which leaves files read/only until they are checked out for modification. The Perforce Eclipse plugin will check out a file and make it writable as soon as someone edits the file in Eclipse.
The JBoss Tools Web (Rich Faces) editor does not support code completion if the file is initially opened read/only. You have to check out the file in Eclipse before opening it in the JBoss web editor. Or, if you open the file while still read/only. You have to open the file and make any change so that Perforce checks it out. Then close the file and re-open it. Now code completion works.
The Jboss Tools Web (xhtml) should support code completion on read/only files the same way the java editor does