You can insert an optical fiber into the tissue and measure the attenuation as a function of the distance... Of course you will damage the tissue, but in ex vivo samples you can try to do it.

I preferably  believe that OCT can be a good technique for Dynamic monitoring and quantification of light penetration enhancement in tissue. There have been some experiments and paper on this method while it is a non-invasive one in compare to insertion of fiber optics in tissue.

http://iopscience.iop.org/article/10.1070/QE2013v043n10ABEH014930/pdf