How can I get TextMate's QuickLook plugin to render HAML and YML files?
Dynamic UTIs are assigned to unknown file types so they have a UTI other than "data".

I suggest you define UTIs for file extensions .haml and .yml and use them in your custom QLGenerator. See this post on SU. The linked topic also contains another answer that suggests to use a QuickLook plugin for the public.data UTI every file falls back on.