Hello, @zipit ! I ended up using GetClone() to solve the AddUserData issue.
A little after that, I found myself in a corner again: unfortunately, there's no way to retrieve Userdata selection in the Attributes Manager - that's why I narrow down my Userdata tracks to only those that will be manipulated by using GetCTracks. Among Userdata, Vectors show up with their components, though, and this means matching by name becomes impossible without slicing. Checking the type allows me to know when slicing is needed. This time, it was much easier to pass values as an argument, they were at hand, so I did apply your first solution, effortlessly.
Polymorphism... I haven't heard of it since my C++ days... about 25 years ago, when I thought I'd be a real coder.
This script I'm about to finish will speed up my 3D workflow tremendously.
Once again, thanks a lot!