since there’s no reply of someone who was able to use the opener with the Elcom REK621Y, I want to reply here. There’s a solution! However, I wasn’t able to get ring-to-open.
There’s the module RED114Y which can be installed similar to Ben’s installation here: Elcom BVF-510
I did only connect the wires for ground, opener and bus-connections - not the green one for the ringer, since we cannot get this signal out there as far as I know. The Nuki opener is then set up for generic analogue and everything just worked fine in my case.