Has anyone been able to get a package like SyntaxHighlighter to work with SharePoint wikis?
Not with wikis (haven't tried) but I did get it to work with a Blog site.
I created a syntax highlighter for SharePoint Wiki you will find the code here: http://code.msdn.microsoft.com/SharePoint-Syntax-ac532d71
About the backgrounds you will find my blog post here: http://www.n8d.at/blog/use-syntax-highlighter-in-sharepoint-rich-text-editor/
I've made a blog post about it where I do a step-by-step (also makes formulas work with MathJax, enables redirects and a basic version of templates). But basically you go to Site settings, open the master pages library, click the button to open it in Explorer, download the file seattle.master and add the required scripts in the section (and re-upload of course). Then they're called and ready on all your wiki pages.