I also don't have the ability to install executables. I can recommend two free Excel addins that may solve this problem for people like us. (Full disclosure, I just created the second addin, hence my answer to a 4-year old question).
The first is Andy Pope's RibbonX Visual Designer It lets you edit ribbons or build them from scratch, and has lots of features, including a form with all the built-in button images, a VBA callback stub generator, and others. I've used this for years, and have learned a lot about Ribbon XML while doing so:

The second is one I just created to modify the text directly. As I became more familiar with Ribbon customUI XML, I wanted something that would let me create or modify the XML text quickly, so I created the yoursumbuddy customUI Ribbon XML Editor. It's features include instant XML Validation, the ability to edit installed addins and the handling of customUI and customUI14 at the same time:
