I have no idea what an ostrich egg texture is like. Is it like a chicken egg (which are easy enough to test paint on) or like a turtle egg (more like leather than bone)?
Can you ask an ostrich farmer for a broken shell?
I have painted on chicken eggs using acrylics with a layer of mod podge or gel medium to seal paint
i paint emu eggs and i use acrylics - the only thing you should do is make sure you put a base coat on - usually a gray or black and after the project is done use a clear acrylic to seal the paint so it doesn't chip or fade