Are there any global key combinations suitable or specifically reserved on OS X for user assignment?
Here are Apple's official OS X Human Interface Guidelines for Keyboard Shortcuts. 

The document appears to be a current, authoritative, and complete guide to assigning keyboard shortcuts. It covers both the conceptual basis for using particular modifier keys, as well as the nitty-gritty of exactly which keys and which combinations are reserved system-wide, or likely to be reserved in the future.

Here's second Apple page showing a complete summary of existing keyboard shortcuts.