I have a lot of experience with "spinning props" having done extensive passage making in sailboats, with trawler sized engines. The velvet drives tolerate freewheeling. Some boats have different transmission on counter rotation prop--and that can make a difference in resistance to free wheeling.
I have used "Chain Vise Grips" to stop trawler sized props (actually stop the spinning and then put the Vise Grips on the shaft.). These are less likely to score the shaft. You can use rubber gaket material under the chain vise grips to further protect from maring.
Be sure that your transmissions and packing gland cooling can tolerate freewheeling--if not, then you should stop the spinning. As I remember that Allison and ZF transmissions should not be allowed to freewheel.