I think that the trident had a weight and balance system installed, though reputedly it was not too reliable. I think it used pressure sensors in the oleos of all three undercarriage so that you could also get the c of g. WRT the A320, whilst you can work out the weight from the green dot speed, it is not that accurate, and the green dot is frozen above FL300 anyway. Airbus say that an error of only 0.3 degrees in alpha position can give quite a large error.