This weekend I got SciTE on the netbook with the Linux Mint Software Manager. XML is highlighted out of the box. To add tag completion and automatic quoting of attribute values, I installed support for Lua — maybe I did not need to, looks like Lua might be built-in — and added a /home/mark/.lua with content from the lua-users wiki page on SciTE and XML. Also had to change .SciTE.properties to load the Lua script. I have not tried to extend support specifically for DocBook. (If you have done so, leave a comment.)
SciTE does fine job for basic XML editing, while remaining very lightweight and supporting many more languages. Plus it looks like you can use it on Windows, too.