Unfortunately not. I enabled autostrut visualization, and no autostrut was anyway near the clawed vessel. To make sure no other autostruts on the station interfered I disabled all autostruts on the station and also undocked another vessel with landing gear. Releasing the claw did not work either.
Also thanks for the suggestion to check the console. There is no output at all when clicking Arm/Disarm/Release, but if I click Free Pivot the game displays a "Null reference exception: Object reference not set to the instance of an object".
Playing around with the Object Thrower I entertained myself for 30 minutes throwing small spheres at the Grabbing Unit. Hilarious flexing, vibrations and sometimes spontaneous combustion effects ensued (depending on angle and target). Amazing how flexible and frustrating how powerful the Claw is. And it never, ever lets go, even if everything else around it explodes.
However, I managed to selectively blow up the docking port. Once it's gone the vessel is undocked. After it was free I tried to dock a fuel tank, worked fine - and releasing it worked as well. However, Free Pivot seems to be stuck. Strange.
OK, I will probably go this route and call the docking port an ex-docking port. Easier and cheaper than setting up a new asteroid pusher.