Audio High Pass Crossover Question
Your emitter-follower buffer has a voltage gain that is just ever so slightly less than unity (0.99 or so).  You could safely replace the op-amp in a filter design if the op-amp is configured as a unity-gain buffer.  One example is a Sallen-Key filter.  There are many web-based calculator tools that simplify your effort - one such is here

However, I did run across a web calculator that will do the design as a pair of cascaded RC networks.  It is here