IIRC, the PRU's are memory mapped so you could "talk" to the PRU using that memory. If none of that makes sense to you then you have a learning curve ahead of you before you'll be even close to ready to write software for the PRU to control your servos.
Might I suggest you write your software to control the servo(s) in Python first and see how well it reacts with your OpenCV program. If that's not _fast_ enough then port the servo software to C++ and if that is not fast enough you would then have enough understanding of your project, software development in C and the BBB to start porting your servo code to the PRU(s).