This won't work because it requires user input. People are lazy, and unless you are extremely disciplined I guarantee you will stop wanting to use that input box very soon after installing.
Buttercoin dropped the original JavaScript repository and rewrote the entire thing in Scala/Java. They are not releasing the new software, but planning on white-labeling their new product (very similar to bex.io).
Additionally, some of these libs are unoptimized and performance will change with time.
Either way, since the performance differences are so small, it's probably best to go with what's easiest to work with.
This is a form of security through obscurity. A bot could easily (relative to a positive captcha bot) be created to check if the form fields are visible.
To be fair, a lot of these companies may not have the funding to hire paid employees. It is also often more of a burden on the technical manager to manage the interns in the first place.
It really is good experience for the intern, and if they actually had experience in the first place then they likely won't be applying to unpaid internships.
I like it. Just remember if you use this in production, save $("#progress") in a variable in the ajaxStart function to improve performance (this will call Sizzle one less time usually).