Is a rotating mast head fitting (approx. 10 x 20 mm) that supports the head of the mainsail permitted?

If the size is no bigger than is necessary, yes.

The 2016 CR require rigs to comply with the 2016 CR. The rule relevant to the design of rig fittings has changed marginally. A rotating fitting or a fitting attached to a rotating spar shall be no bigger than is necessary.