I believe that a powered rotating mass on board a spacecraft should result in the spacecraft being propelled in a continuous increasing velocity towards the source of the gravitational field.
To help visualize this conceptual idea, consider this simple drawing of a monster truck that is moving in outer space towards a star.
Say that this truck has four 72" solid metal discs instead of rubber tires and there are two electric motors, one that is turning the front axle and one that is turning the rear axle.
As the four discs are rotating, at any one point in time the atoms that make up the top half of the discs (white colored areas of the discs shown in the picture) are going to be moving in the direction of the star while the atoms in the bottom half of the discs (gray shaded areas shown in the picture) are moving away from the star.
Since the atoms within the gray shaded areas are being forced away from the star by the torque of the electric motors, this should cause the motors to be propelled faster towards the star via Newton's Third Law of Motion, and this should continuously increase the spacecraft's velocity towards the star. This should occur because these atoms are being forced back against an ever-increasing gravitational field strength as the spacecraft moves closer to the star.
Also, there should be an imbalance of centripetal/centrifugal forces around each disc because gravitational pull is stronger at the leading edge of each disc than at the trailing edge of each disc.
If this is what will occur, then an ideal application for this type of gravity-assist propulsion is that the continuous increasing velocity towards the star will reduce the travel time for the spacecraft getting to its destination, especially if its intended purpose is to slingshot around the star as a means of trajectory change towards its destination. As the spacecraft begins its slingshot around the star, its rotating discs will be stopped so they do not decelerate the spacecraft as it travels away from the star.
Will a spacecraft accelerate in a gravitational field via the use of a powered rotating mass(es)?
Although this gravity-assist propulsion will not work with rotating discs, it should work, in theory, if flyweights are used. See revised drawing below.