If the 757 Wx radar is the same as the 767, I would be very cautious in using it as a means or aid to avoid terrain.
I was never able to obtain any usefull information from this wx radar in ground mapping. Example: over water, I an a ship but with the wx radar unable to pick it up. Another example coming into Montreal, where the St-Lawrence River and its bridges are easyly seen and discrimated from the river, same result, unable to see them on radar.
I agree that with some wx radar ( I have seen some) you can do what has been said. But, make sure that you have tried it on a CAVU day to learn how to interpret the info.