I've gathered there is a population of "developers" who refuse to demonstrate the ability to code at any stage of the interview process.
(The OP is definitely not part of that crowd, given his willingness to submit a pull req to some open source project.)
My current job, and every previous development job has clauses about outside development, and what internal hoops have to be jumped through. I'm compensated for that restriction, so I don't mind.
Even lacking those restrictions, contributing to open source projects just isn't really my thing, and my personal projects are just that -- personal, and are usually ugly, hacked together in a hurry and never revisited solutions to specific problems, not resume filler.
If you can't find some way to determine if I'm a good fit without looking at github, having me do unpaid work, or playing trivia games, you need to revise your hiring policies.
Why have we, as a group, allowed the creation of this requirement, official or otherwise, that we have a public portfolio of work we've done on our own time?
In the real world, some of us work on highly proprietary code bases, where putting any code up online could expose the risk of future lawsuits or prosecutions, certainly this has already happened to individuals in the industry.
Thankfully in the real world there are some sensible and mature interviewers who are able to look beyond the heinous crime of not having a Github presence.
For every one person who is great but can't publicize any of the good code they write, there are a lot of people who simply suck. Differentiating between them is expensive (read: hours interviewing the good candidate, many more hours interviewing bad candidates) which is why the people with a good github/bitbucket/ftp server full of code are at the top of my list. If they all bomb the interview, maybe I'll move down the list to the people who's skills are unknown.
This simple reality is why one should demand more money from employers who impose restrictions on what one does outside of work.
My point is that if having a github account becomes a silly barrier like having an education from top school, it kind of defeats the point of being fair whilst also acquiring good people.
As someone who is now interviewing for a new job. I've had three interviews this week ask pointed questions on github contributions including requests for me to change things on the spot ( or talk through a change that might occur).