Seems like an arduous project: The viewing screen of the SLR is ground glass. This is liken to etched glass (roughened surface). Its job is to provide a toothed translucent surface that captures the aerial image produced by the camera lens. The mirror is just a device to bend the image forming rays upward so they are square with the viewing screen. The etched viewing screen will be a poor cover glass for your LCD screen. The split image viewfinder and likely some micro-prisms are a feature of the viewing screen. They present a split view and or a super blurred view of a snippet of the image projected by lens. The image superimposes or snaps sharp depending on the angle of the arriving image forming rays from the lens. There is no chance they will function close haul to an LCD viewing screen. Additionally above the ground glass viewing screen is a Fresnel lens. Its job is to mitigate a vignette that is likely, fashioned by the camera lens.

Above the viewing screen and Fresnel lens is a “roof” prism. This produces multiple reflections of the image grabbed by the viewing screen. This image is upside down and inverted left to right. The “roof” prism corrects this view. Next is a eyepiece lens. This is simply an achromatic eyepiece (magnifier) that allows the human eye to focus on a screen only a dozen millimeters away. Seems to me, converting an SLR for your task, complicates the job. However you can do it, just strip away the reflex mirror and the view screen. Lots of extermination will be required.