This is honestly a case of "Don't bother with the real app!" Have whoever is developing the UI send you images or e.g. an XD design file and integrate it natively. This also avoids issues with the phone screen illuminating the hand and complicating the roto work, as effectively it can simply be turned off/ covered with a matte green foil or similar. That or you create a custom simulation packeage with the XD file and have it run on the mobile in place of the real app. solves half your problems without any convoluted extra procedures.