We present an approach to project source directivity variations onto a binaural room impulse response measured with an omnidirectional loudspeaker. The approach consists in applying frequency-dependent gain weightings to different time windows of the impulse response, issued from the desired source directivity pattern and orientation. The end goal of the research is to achieve plausible directivity pattern perception in auralisations, thus allowing one to mimic the rotation of a sound source with a given directivity pattern in mixed reality environments. We present the first step toward this goal, examining the perceptual threshold of analysis window size on recreating authentic room rendering of a rotating directional source. Perceptual listening tests were conducted to assess the impact of window size on the perceived authenticity of auralisations made with the proposed method. The results of this preliminary study intend to inform the ongoing development of the approach, which will next be extended to allow any arbitrarily imposed directivity pattern using perceptually motivated principles. The generalisability of this approach across different source-receiver configurations in different rooms is also discussed.