The title screen/menu BG can contain some segment of the in-game sprite, such as closeup of the eyes or its silhouette. It would also be OK to use a lineart-only version, of the in-game sprite as well (as long as it's the lineart layer from the in-game sprite). A new pose, or a completely different character however would go against the idea of a single "sprite", despite it being a baked in part of the (allowed) special menu background. If you had a stylistically unrelated representation or suggestion of a character in very abstract form (such as a simplified logo or chibi/mascot) be a part of the menu background, then that would be fine.