Hi @flyingrobin and thanks for your query. The Interaction Library (Core SDK) does support masks. You can specify a rectangle and a bitmask as referenced here @ https://tobii.github.io/CoreSDK/api/Tobii.Interaction.Model.Mask.html
I am afraid we do not have further samples on implementation, but hopefully this is enough to get you in the right direction.
Please try it out and let us know how you get on. Best wishes.