After much experimentation, you can indeed stretch particles by moving their velocity vector away from their position vector with the STREAK flag set and the MOVE flag not set.

However, the stretched particles end (moved by vel_xyz) cannot move off the edge of the cameras view until/unless the position end of the particle catches up. It looks as if the particles leading edge hits the edge of the veiw and stops. Once there, you can stretch compress the stretched particle just by panning the camera!

So sadly, GS still cannot stretch particles to emulate effects like Unity3D's particle renderers "Velocity Scale".