I'd like to add some simple block diagrams to my GitHub wiki. Is there any way to do this without embedding images?
You might try Figurepool to create a diagram, generate a Markdown code snippet that it provides and just copy paste the snippet into the Github wiki. Check out this blog post: http://figurepool.com/blog/Embedding_images_and_diagrams_in_github_wiki_pages_and_issue_tracker.html.
At the moment umlet-github does not support GitHub wiki's however I'm working on a project to solve some of the larger problems in this area at the moment and will update this answer to reflect developments.