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Edit: Please refer to Mikey's answer below.

Yes, that is allowed! Your main menu counts as the User Interface.

MAIN MENU/TITLE BACKGROUND:  The main menu or title screen can be different to the in-game background.

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.