In Chapter 8.1 of XSL 2.0 requirements there is stated: "Add support for adding annotations that appear in the output, e.g. comments or text highlighting in PDF."

If annotation objects will be supported and XSL-FO processors will be capable to insert them into PDF output, I would suggest to add new attribute to changebar object which could switch on/off rendering changebars as line annotation object. Changebars in form of annotations can bring some advantages. In Acrobat all PDF annotations can be displayed as list and user can easily jump from one to another, which is impossible in case of drawn line. Although XSL-FO processors could produce changebars as annotation by default without specification, maybe such attribute would be helpfull for any environments where annotations could be potencial problem.

Jumping between annotations is not available in free Reader yet (if document has commenting disabled), but there is Feature Request in Adobe forum: http://forums.adobe.com/thread/425936