After some months of trials and (mosty) errors, I may have found an intermediate solution, while still hoping the Custom Instruments Definitions to appears in one of the forthcoming versions of Dorico.
As Pat describes here above, I ended up using custom keyboard and pitched percussion definitions for my ensemble libraries. For example, VSL’s BBO Andromeda is Synthesizer, and BBO Regulus is Electric Organ. These are definitions that I would not use while sketching.
When applying the BBO playback template, there is not interference, and staves are not reset by the wrong endpoint configuration. When sketching, I can simply call a library with the nickname I assigned it in the general map, so that I know that invoking a Synthesizer is, in reality, calling for BBO Andromeda.
And now, another problem on sight: with all the ensemble libraries released during the latest months, I’ve nearly finished all the fake definitions in Dorico!
Paolo