Question: Are there any half-official repositories for CentOS 5 with Python 2.7?

EPEL provides only Python 2.6

Answer: 
I made an RPM for this a little while ago. You can find the source RPM here

<ftp://ftp.pycopia.org/pub/python/RHEL51/python2.7-2.7.1-1TH.src.rpm>

Just do the usual `rpmbuild --rebuild ...` to create the binary rpms. It does not interfere with the installed 2.4 version, it coexists with it. but you do have to put the specific version in your scripts: `#!/usr/bin/python2.7`. But if you use setuptools invoked with that version then it happens automatically.
Not that I am aware of. However, you can find `.spec` files on the net to build parallel python versions where all the RPM files are named python27-whatever etc. Then the default `python` executable is still your vendor one, which should **not** be changed or you may break very important things, like `rpm` itself (I speak from experience).

Note that starting with RHEL5 (I have no experience with 6), `rpmbuild` will "precompile" python in any RPM. It's a known bug that they refuse to fix that it will mess up the `%files` section. Anyway, the problem is that the files will be precompiled with the stock OS python, which is really annoying because then you get "Bad Magic Number" errors if you try to use them. You need to change some RPM config scripts, but I don't have the info in front of me.