Thanks Michael, that's what I feared.

My customer now tries to get it done in Catia, a software I know nothing about.
Somehow weird that this should be so rare, sounds like a very common need to me.

If all else fails I would have to rebuild the models in Polygons with Subdivision Surfaces, where you can create continuity more easily.

It's really too bad that on these models, imported from Catia as Step, I can't even fillet a single trivial edge, no matter what mode in MoI.

Cheers and thanks,

Tom