I was interested in such an experiment myself, but had not enough experience and resources to hold one. Missed one held by TWU in 2019.
Long story short, no difference in rebound velocity if a ball is blocked by free-to-move racquet vs hand-held racquet. Just too short contact time for the frame to transfer impact all the way to the hand and the resistance - back to the top. All comes down to precise sweetspot striking. So no body mass contributing directly during contact - just the hoop and it’s speed.
-Good find here but maybe more to this than meets the eye.
The best COR (as you see in the chart at 20-25 cm or about 9") is closer to the throat than the areas normally called the sweet-spots. More COR is more power with less hoop speed. Since hitting out at the sweet spots can cut power to about half, then why not hit nearer to the throat which gives more power and control for less effort on certain types of shots? To get as much power further out on the strings, the racket must be rotating the tip around fast enough to compensate for less COR. That rotation if over-done or improperly done can create several stoke problems.
Congruent Tennis Creator PTR Instructor