K-On! makes references to P-MODEL through the five main characters' surnames. Many K-On! fan works make reference to this, with examples of fan works making references to Hirasawa Susumu using Hirasawa Yui found in posts such as post #481261 and post #480735.
He once made mention of Yui on his Twitter:
Are you mistaken? I am Hirasawa Susumu, NOT Hirasawa Yui.