As I have mentioned before, the CustomUI editor is far from perfect. It does allow adding of image files and to validate the XML, but the editing experience is absolutely awful.
My favourite editor is TextPad, and one of the useful features of TextPad is the facility to incorporate syntax files that are file type dependent, for instance a Visual Basic syntax file for .bas and .vbs file types. The syntax files can highlight elements of your files which assist in entering script as well as making the script more readable.
Although I rarely use the CustomUI editor when I am developing Excel ribbons these days as I autogen everything, my autogen doesn’t yet handle Word or Access ribbons. As I still don’t want to use the CustomUI editor, I built myself a CustomUI syntax file for use with TextPad. I can now edit my XML in TextPad, and copy and paste into the CustomUI editor when it is ready. I find this much more productive.
It is still necessary to setup TextPad with a document class for these files, setup the keyword colours and so on, but if you think it could be useful, you can get it a copy via the RSS feed, or download it here.