Perhaps there is already a solution
There are several Sports watches on the market with optical detectors integrated into the body of the watch.
A company called Scosche has produced an arm worn band with an optical detector which uses an extra LED to transmit light of a different frequency which penetrates further into dark skin.
I would have thought that Apple had taken this into consideration during the design / testing stage, but perhaps not.
Biting the hand that feeds IT
