I reached this page when I searched for information about openssl being keg-only. I believe I have understood the reason why Homebrew is taking this action now. My solution may work for you:
Use the following command to make the new openssl command available (assuming you have adjusted PATH to put /usr/local/bin before /usr/bin):
ln -s /usr/local/opt/openssl/bin/openssl /usr/local/bin/
When compiling with openssl, follow Homebrew's advice and use
-I/usr/local/opt/openssl/include -L/usr/local/opt/openssl/lib
Alternatively, you can make these settings permanent by putting the following lines in your .bash_profile or .bashrc:
export CPATH=/usr/local/opt/openssl/include
export LIBRARY_PATH=/usr/local/opt/openssl/lib