Starting at the end of 2025, the Vera C. Rubin Observatory and its Legacy Survey of Space and Time (LSST) will survey the visible night sky every ~3 nights down to ~24th magnitude in 6 broad-band filters. Rubin Observatory will completely change our view of the outer Solar System with the LSST expected to discover over 40,000 new outer Solar System minor planets. This is about an order of magnitude more objects than are currently known to reside in the Trans-Neptunian region. The LSST will place the best constraints on the existence of possible distant unseen large planetary-sized masses, within the regions of the Kuiper belt and Inner Oort Cloud, that may have sculpted in the past or are actively shepherding the orbits of distant minor planets. We use Sorcha, a bespoke open-source python Solar System survey simulator for the LSST, to examine the prospects of LSST discovering these proposed planets. Sorcha is optimized for the scale of the LSST discoveries (~5 million Solar System detections/ ~1 billion individual photometric and astrometric detections at the end of 10 years). Sorcha can take any input orbital population and bias it to what the LSST would have detected utilizing a real or simulated LSST pointing history and observation metadata combined with the latest information about Rubin Observatory's expected performance and the Rubin Solar System Processing pipeline's detection efficiency. I will present the prospects and opportunities for Rubin Observatory to discover and test various proposed “Kuiper belt planets” or planetary-sized masses residing in the Trans-Neptunian region and beyond, including the Planet Nine model. I will also discuss the parameter space that the automated Rubin Solar System Processing Pipelines will be able to search and constrain, expectations for the first few years of the LSST, and opportunities for additional search algorithms.