In WebGL 1.0, only opaque multiview framebuffers can be supported. Texture array multiview framebuffers are available starting from WebGL 2.0.
You may have misunderstood the native OVR_multiview spec a bit. The baseViewIndex only affects which texture layers get affected by drawing commands. There can still only be one attachment per attachment point. So if you call
int baseViewIndex = 7;
int numViews = 2;FramebufferTextureMultiviewOVR
(target, GL_COLOR_ATTACHMENT0, texture, level, baseViewIndex, numViews);
it means that texture layers 7 and 8 will be drawn to. This should be evident from the pseudocode in the spec.