OpenCV is an independent set of library that has nothing to do with ROS. Theoriacally It can be compiled and run on any operating system like Android, or iOS.
When you talk about px4 stack, there is Nuttx version and linux version. Not sure which one you talking about.
In theory OpenCV does can be ported to run on Nuttx, assuming you have enough powerful processor. But i guess lot of porting work will need to be done in advance.
If you are talking about Linux version of PX4 than things will be much simpler as download and install.