I would like to know how I could retrieve definition property such as human task priority before it was started. I know there are query API to search plan item instances but I did not know how to read task properties which was not yet started.
My guess would be that you can’t do that out of the box since a lot of the fields on the tasks are expressions that will be resolved at runtime. What you could do if you know for sure that a field will always be filled with a string value is to navigate the BPMNModel to find the task you are interested in and extract the priority value from there 
Hope it helps!