I debugged the process of loading the model and found that bench.cc switches to my local _build folder at some point and uses the model file located there. I opened a ticket [#155] for this issue so that at least a warning is issued or, better yet, the correct file is loaded from the outset.
However, a current workaround is to rename the modified model. When I named the model Severi2.model, created the dynamic library and then loaded it, adding the Ko parameter was effective.