Is it possible to add a bracket to a group containing instruments which do not belong to the same category?
One instrument is a Sampler and belongs to the category Electronics the other is a Piano and belongs to the category Keyboards.
Though I can easily group both instruments I can’t find a way to get a bracket for this group.
How can I achieve this?

It is not possible at the moment, I’m afraid, but it is of course in our plans to improve the flexibility of Dorico’s bracketing in future updates.