Normally a playback configuration will map to the instruments listed in the score. But of course there’s nothing to stop you simply creating playback using Pianoteq for every instrument you use. For instance if you wanted to create a string quartet with every instrument heard on the piano, you open four instances of Pianoteq (or use one instance in VE Pro if you have it) and map these to the string instruments in the Play window. You can save any one of these as a Pianoteq endpoint and use that as the basis for a playback template. For orchestra, you again need to map any instruments you might want to use to Pianoteq playback.
Basically, Dorico needs to have in the PB Config a list of all instruments you might want to use mapped to the piano – this is how they work. Unused instruments in the score are simply ignored. Effectively I’m saying pretty much the same as @benwiggy here and this certainly works.
I’m assuming that in your test, you set Pianoteq as your default Playback Template? I find doing that, after “load sounds for unassigned instruments” does indeed work as expected. However, say you wanted to use a flute instead of a violin from my quartet example, Dorico doesn’t know what to assign a flute to as it isn’t listed anywhere. I’ve tested this and the result is as expected.
The ought to be a simpler way and perhaps someone will chip in with something we’ve missed. But the above does work with my tests.
Of course something like just creating a piano reduction of a score could be worth exploring in some situations as this is the only practical reason I can think of for what you’re wanting to achieve. But then, I’m not you and I’m sure you have your own reasons!