You are right, SSLRequire is the most interesting
Fun by side, great howto.
Most of the manual work you can get 'automated' or 'built-in'. The apache 2 plugin contains the exec call's for the certificate creation, and using opensssl ca -batch does not ask any question, therefore it can be built in completely.
Back to the most interesting ...
.. what came to my mind is now, to identify ISPConfig 3 users with certificates