I always seem to accumulate vegetable scraps faster than I do bones, so would it be possible to create a vegetable stock in a pressure cooker, to then create a chicken /beef stock at a later date when the bones are available? Specifically, would there be a considerable loss in flavor, or would there be a better way to go about this? 
I'm not certain what you are asking?  Are you wanting to start a vegetable stock and use it as a base for the bone stock at a later time ?  IMO that would work.  Do a vegetable stock when you have enough vegetable scraps, portion it and freeze it.  When you have enough bones, do the stock using the vegetable stock use made earlier. 