For the Linux kernel, that's how development is done already, for quality control and bloat reduction. Nobody can commit by themselves, it takes at least three people to get a change into mainline. Each developer has their own copy of the tree into which changes are pulled, so they can see all changes that are made, and who made them.
For each part of the kernel, there are a number of people particularly interested in that bit who watch it and work on it. For example, the people making NAS and SAN devices and services keep a close eye on the storage subsystems. Myself, I watch the cm storage stack generally, more specifically LVM, and even more specifically snapshots. There are a few dozen people around the world with special interest in that particular part of the code. No backdoors will come in without some of us spotting it. What COULD happen is that some code could come in that isn't quite as secure as it could be.
It just so happens that I'm a security professional who uses advanced Linux storage systems for a security product called Clonebox, so that's at least one security professional closely watching that part of the code. Thousands of others watch the other parts.
It's convenient that a lot of the development is done by companies like Netapp, Amazon (S3) and Google. You can bet that when Amazon submits code, Netapp and Google are looking closely at it. When RedHat submits something, Canonical will point out any reasons it shouldn't be accepted.