It would be awesome if the repository would host two versions of the nginx package, one with SPDY for 6.5+ and one without it for older versions.

You would need to build two versions, but it would be extremely easy to add this support, you would just need to create symbolic links 6.1, 6.2, 6.3 and 6.4 to point to directory 6 (for the non-SPDY version) on this URL http://nginx.org/packages/centos/6/, and create a new directory 6.5 which will host the version with SPDY.

You would also need to change the nginx.repo from:

http://nginx.org/packages/centos/6/$basearch/

to:

http://nginx.org/packages/centos/$releasever/$basearch/