Display syntax selectors with unicode characters (aka Vim's conceal)

In Vim 7.3 they added a really nice feature allowing to display some syntax regions (selectors) as configurable unicode characters. It makes much easier to read some languages, like the following example for Javascript where "function" has been replaced by "𝑓", "this." by "@" and "return" by "↩" .

With the recent inclusion of code folding, which allows for "inline" folds, I think the editor is ready to implement this feature without much effort. 

I've tried to implement a plugin for this but I'm lacking one API function, which could work just like View.fold but allowing to set a custom character and syntax selector for its display, instead of the currently hardcoded fold icon.