This shouldn't be a problem. At least not when using the GUIDRV_Lin driver. emWin manages the framebuffer content. How these data get to the display is not part of emWin itself. In general this is done by the on board LCD controller of the STM32. If you configure the controller to 'feed' the HDMI interface it shouldn't be a problem to use it. The same is possible with the MIPI DSI interface. Unfortunately, we don't have a sample for this.