Back door route to top research schools? Is there a way to gain admission to these institutions by getting paid experience prior to applying? 

I have a friend who graduated from a top 150ish school in the US and did moderately well (biochemistry, 3.8 GPA). He had some research experience on hand during his undergraduate career, but didn't have publications. He had about 2 year's worth of research, and maybe ~5 poster presentations at conferences. He did a summer internship at a medical research institution where he now works as a research assistant (after he graduated). After catching up with him and talking, he admitted to me he could not get PhD admissions to top tier universities with rudimentary statistics straight out of underad. He spent about 2 years working at said medical research institution, got I believe 4 papers under his belt, took his GRE, did moderately well but not amazing, and applied. 

&nbsp;

Now he's been receiving admission into institutions such as MIT, Stanford, etc. Institutions by his own admission that he could not get into with a snowball's chance in hell before he started working/publishing. I was curious, what was the catalyst? Did paid experience prove he was worthy of the skills and not cheap undergraduate level (aka solidify he was "the real deal")? Was it the papers? Was it him now riding the prestige of his employer and not his undergrad? Was it him being 24/25 and a little more mature/realistic in interviews as opposed to the bulk of his 21 year old competition? Was it showing graduate schools that his 2 years of succeeding at his institution meant he's more likely to succeed into graduate school life? This whole idea got me curious if it's possible to massively increase one's chances at admissions to higher tier or even above average graduate schools in the sciences if one is patient and willing to wait enough to gain experience once they're done with their undergraduate career. Academics of SE, could I have your input?
I wouldn't call this a backdoor route.

It takes time to get quality publications and letters of recommendation yet undergrad goes by fast. Your friend also wasn't from a top university which makes these opportunities even harder to come upon.

There are a lot of details missing here (what were his letters of rec like, quality of publications, etc.) but it sounded like your friend did some good work and got accepted because of it. Whether you do this work during undergrad or after you graduate, doesn't necessarily matter.