Hi Daniel,
there are many different versions of homogenization. Typically, you calculate the coefficients for the homogenized equation either by pencil and paper, or by solving a pde on a unit cell numerically. This is your job.
Then you can directly specify the homogenized equation in NGSolve.
You can have a look into our papers with Karl on multi-scale finite elements for Maxwell equations:
www.asc.tuwien.ac.at/~schoeberl/wiki/index.php/Karl_Hollaus
best, Joachim