Graceful JavaScript fallback for external web service failure
I like the idea, from a once over:


Since you use an IIFE I would declare sencha_path_identifier and sencha_path_regex outside of the constructor, then you can access them without specifying this.
I would do either function SenchaSRCFallback() { or var SenchaSRCFallback = function SenchaSRCFallback() {, anonymous functions are a pain to troubleshoot
Why do you require the caller to call init()? In my mind this call should be done within the constructor.


This is something I might borrow at some point.