To install the client hotfix during Task Sequence men have to follow the same way as when installing client hotfixes for SCCM 2007. So you simply have to use PATH option in your "Setup Windows and ConfigMgr" step.
The PATH line will look like
Where %PACKAGE_ID% is SCCM Package ID of the "Configuration Manager Client Package"
There is one small thing that should be fixed before this is actually going to work ...
When installing a hotfix for SCCM 2007, the client patches were automatically written to
%SCCM INSTALL DIR%\client\%ARCH%\hotfix\KB_NR\... and the only thing men had to do was update client install package and specify necessary KB in the PATH.
In SCCM 2012 hotfixes are no longer written to that path, but to %SCCM INSTALL DIR%\hotfix\KB_NR\...
To make the above story work we need to create subfolder hotfix under %SCCM INSTALL DIR%\client\%ARCH%\ and copy the corresponding client MSP's to there... and this is it (DO NOT FORGET TO UPDATE CLIENT INSTALL PACKAGE)