Sear pork belly before or after cooking in a pressure cooker?
If you sear the meat in the pressure cooker before putting the lid on, then the flavoring from the seared tidbits and caramelization will assist in bringing a fuller flavor to the finished recipe. 

This is the same technique I use when I cook something in my Dutch Oven (as a substitute for my slow cooker). Instead of trying to transfer everything into the slow cooker, I simply slide my Dutch Oven into the oven and set the heat (for the equivalent of high or low). 

I don't see any reason that the same principle shouldn't work for you with your pressure cooker.