@PaulWalmsley hello,
Well, some improvement could be made here in order single expression to trigger multiple Keyswitches to multiple channels.
Here is my suggestion:
In case we need to route a Keyswitch to an Absolute, or Relative Channel we could select it right after the velocity section, as shown on the screenshot above. If we don’t need to route the Keyswitch, then we could just leave the Channel section blank. The same should be valid for Control Changes and Program Changes if they need to be routed, or not. 
This would make the Expression Maps a very flexible tool and will reduce the mapping work a lot.
Even in combination with this suggestion of mine - [FIR] Expression Maps Improvements for Dorico 4, we will be able to easily combine and blend instruments from different libraries in order to get richer and more natural sound out of the VIs. For example Divisi in 4 voices could be performed by 4 different libraries by using only single expression map. Imagine I would like to create custom large orchestral template which includes BBC SO Pro, Orchestral Tools - Berlin Bundle and CineSamples - Orchestra Bundle, which is common practice among the film composers to use multiple libraries within a single template. So, would be more logical and easier if we are able to create single expression maps set per section of instruments, instead of creating separate sets per library.
As you can see, on the screenshot. the Staccato is selected and routed to a three different Keyswitches and three different Channels. In this case we might get simultaneous staccato playback from three different Violin libraries loaded within the same Kontakt instance.
Sometimes we combine libraries just because one library doesn’t have the articulations that another has, or just because we prefer how some articulation are recorded and sampled in one library and others are sampled in another library. Combining techniques from different libraries directly inside Expression Maps would be great without the need to change Note Voices, nor Ports. You could even add Port section next to the Channel!
I hope the team would think about to make such small improvements, which will make the work with VI libraries and the E.Ms. creating process far more easier in Dorico!
Best regards,
Thurisaz