I'm stuck on the same problem.
We use a lot of end/weld plates to connect beams to pre-cast concrete tilt panels in large construction. I wanted to create a custom steel connection for the beam -> concrete wall interface and use the new Dynamo player to send those everywhere they are applicable based on ranges.

The bad news is according to Autodesk's help page, custom connections only work with steel beams and columns. Along with many other limitations.

Worse, even if the custom connection is only attached to the beam end, applying a structural connection to the framing member removes the end grips and automatically snaps the beam end to the center of the structural wall.
The "Change Reference" tool also stops working.
Leaving the only way to align the beam properly with the wall surface to zooming in and adjusting it manually, which again defeats the purpose of this whole exercise.



I've been thinking of formulating an "ideas" post for addressing this in future updates because at the moment this severely limits the possibilities for custom connections.