I searched but couldn't find an answer for this -- is there a way to copy a transition, e.g. an affine-composite for which you've done several keyframes?

No matter what sequence of events I try, "Paste Effects" triggers the error message "You must copy exactly one clip before pasting effects".

More generally, is there a way to build a reusable library of e.g. Affine Composite transitions, with the keyframes (and all customized attributes at that keyframe, e.g., opacity, location of object, X/Y/Z values, etc.) intact, such that you could subsequently apply the transition, keyframes/path/rotation intact, to another object? Unlike with the effect stack, there's no apparent way to save a transition to disk ...

What I'm trying to accomplish - a stock title sequence with text on top, pictures and videos moving/rotating/fading in/fading out in the background. I can set it up easily -- but then, I would like to copy it and simply replace the foreground text/background images.

Thank you!

As as I know it this is not yet possible.
However a feature I would prefer would be something like replace clip. You could then save a sample setup of what you described above and simply replace the foreground/background images with others and save the whole project under a new name.