How about aircraft control rods and cranks with a separating joint below the baseboard using a super magnet? The West Group layout has used aircraft control gear - it was fitted about 40 years ago and worked directly from the lever frame. Has worked well for all these years. There are a couple of runs that go over the joints and when the lever is at normal and not pulled the rod is fully back and the adjoining rod has a spring to return it to the baseboard edge, so that when the boards are joined the lever pushes the rod into the next board and therefore pushes the rod on to the rod on the next board and compacts the spring in the process.
Using a super magnet instead of fitting a spring. All you would have to do would be to double check that the magnet will have coupled up to the other steel rod when the boards were put together, if that makes sense.
Unfortunately I have no photographs of the underside of Calderside - I will try to take some at some point, but it will not be this week as our AGM is on Tuesday evening and we are going to have a meal beforehand.