Conventional boron nitride (BN)-based composites are often limited by high porosity and long processing cycles, which severely restrict their mechanical performance and engineering applications. To overcome these limitations, a novel organic–inorganic hybrid matrix combined with a hot-compression orientation strategy is proposed. Liquid-phase polyborazylene (PBZ) was employed to promote the rearrangement and alignment of h-BN lamellae during pressing, and subsequent pyrolysis yielded a dense and continuous BN matrix with in situ precipitated nanocrystals serving as load-transfer nodes. This process significantly enhanced densification, reduced porosity, and enabled the construction of an ordered lamellar structure. Unlike conventional PIP processes that typically require ~10 repeated infiltration–pyrolysis cycles, this method achieves dense composites in a single pyrolysis step, effectively avoiding fiber damage and greatly shortening the fabrication cycle. The synergistic effects of aligned lamellae inducing extended crack propagation paths and in situ nanocrystals providing additional load-transfer mechanisms effectively improved the mechanical performance of the composites. The proposed PBZ-assisted hybrid matrix and lamellar-orientation design strategy offers a new and efficient route for developing high-performance BN-based composites with excellent wave-transmitting and thermal protection capabilities.