The readme.txt file in the PCBuild folder in Python 2.7.11 and 2.7.12 sources both contain instructions on how to configure to build a different OpenSSL version using PCbuild\prepare_ssl.py, but this file is missing on both versions.
It can be found at PCbuild/prepare_ssl.py in the default branch. Copying it into the 2.7 branch wouldn't hurt I guess, and I'm okay with having build scripts in there that depend on Python 3 ;)
It would need someone to volunteer to port and maintain it. I don't believe any of the current team want to maintain two different versions of the script, considering we all use Python 3 and have it available when updating the version of OpenSSL used.
You can also use PC/VS9.0/build_ssl.py which was the progenitor of prepare_ssl.py. The project files in PC/VS9.0 will use build_ssl.py by default. FTR, I'm good with copying prepare_ssl.py to 2.7, and even with ensuring that it's 2.7 compatible (it probably already is), but I want it to match between branches and it must be Python 3 compatible.