Hi,
I don't think this is a Docbook 5 issue, but an issue with the XSL-FO processor. You might take a peek inside the generated .fo file to see if your narrow space character is there. If so, then the XSL stylesheet is working and the XSL-FO processor is not handling it. What XSL-FO processor are you using? The FOP processor included with Oxygen?
There are a lot of space characters in Unicode (see
http://www.sagehill.net/docbookxsl/SpecialChars.html#SpecialSpaces), and not all XSL-FO processors handle all of them. It may be that some processors expect to find the character in the current font.
Bob Stayton
Sagehill Enterprises
bobs@sagehill.net