As stated in another recent post, spheres moving at significant speeds actually "appear" rotated but still spherical. This is a combination of Lorentz contraction and the finite speed of light. (I wish I could find the reference to the erudite paper that presented this surprising fact). I am not sure whether the same would be true of the EH of a BH as light would not be emanating or being reflected from this surface. I think answering this (as with many such questions about physics involving general relativity) are very difficult to answer and could take a long time to calculate by someone with the mathematical knowledge and ability to do so. It doesn't stop the rest of us speculating though :-), but we should treat any answers with scepticism unless backed up with the maths. You can't really just guess...

...but, my guess is that it is affected by Lorentz contraction so would still appear spherical as does any sphere :-)