My solution would be to setup a standard exam that would result in a score. This score would be valid for a period of 6 months, which would help candidate applying to multiple companies at once using the same score. You would have the possibility to re-apply to the exam several times until you'd land a good score. Your score would expire after 6 months. This exam would be hosted by several skilled engineers from different companies. They would be trained in evaluating technical skills and would be specialized in different areas.
When you'd apply for a job, there would be a place to enter your score or a way for the company to pull out your score. Then they would trust that score and not run any technical interviews during the hiring process.
Instead, they would only focus on cultural fit. All you'd have to do is to chat with the team about work, life, etc. Of course, you'd get invited only if you have a certain score. By focusing on cultural fit, this would increase employee retention. Also, this would balance out the referral process and give an equal chance for everyone. My score would be comparable to your score in a meaningful way.
Long story short, we need a GMAT for software engineers in order to normalize the process.