Well mass flow would be mass times speed. If you can weigh the contents of the pipe and simultaneously measure the velocity then you can work it out.
I've seen flow meters for large scales, like coal on a conveyor belt. There's a section of belt that's supported by a scale and it's not hard to measure the speed of the belt. Powder in a tube will be a lot more difficult because (1) is it really weighing on the tube? and (2) What if the speed is not consistent across the tube diameter.
If you can drop the powder onto a small conveyor belt then you'll have a much easier time of measuring the mass flow.