Does Xcode support anything akin to Visual Studio style #region directives for arbitrary code folding?
No, you can only fold code on various defined scoping levels in Xcode.
You can use little tricks to make navigating via the function menu easier, though.
#pragma mark
Allows you to create a grouping where the label following mark will show up in the function menu. If the label is a hyphen, a separator is inserted into the function menu.
Also, the following labels in comments will show up in the function menu:
// MARK:
// TODO:
// FIXME:
// !!!:
// ???:
Obviously since #pragma mark is not really portable, if you're building a portable application and need it to work with a compiler that doesn't just ignore #pragma directives that it doesn't understand, the comment-style mark is a decent alternative.