It'd be nice if there was the equivalent of a "null bus" that you could direct a send to, to suppress the silent-bus message when a silent send is actually desired. Example: using a track solely as a sidechain key. 

We do have a null bus - its the bus called "--none--" which internally is really a bus that routes to nothing.
In your example if the track is solely a sidechain key why do you even need a send? Why not set the track output to the sidechain?
When a send is orphaned SONAR sets its destination to be the null bus. So the way it is today unfortunately we have no way of knowing if this is intentional or not.

My original warning states that my "Vocals" bus is assigned to a silent bus. This is not true. My "Vocals" bus is assigned to my "Master" bus and there is one send on that "Vocals" bus that is assigned to a "Reverb" bus, which is assigned to my "Master" bus.
I have had this happen on a couple other projects, also. On one of them the warning references a bus on a completely different song/project????? 

I've had this too and it's more than a little annoying. Tbh I just reckoned I'd done something wrong so I deleted the bus and created a new one. But if it happens again I'll be having a much closer look..................

what kind of reverb is on that bus ?
a 3rd party reverb ? possibly the plug in is not opening in synch as the project opens.

rodreb
My original warning states that my "Vocals" bus is assigned to a silent bus. This is not true. My "Vocals" bus is assigned to my "Master" bus and there is one send on that "Vocals" bus that is assigned to a "Reverb" bus, which is assigned to my "Master" bus.
I have had this happen on a couple other projects, also. On one of them the warning references a bus on a completely different song/project????? 

Put your cwp file somewhere for download and I will tell you why its reporting it has a silent bus :)

Charlie : Breverb
Noel : Thanks! I have a busy tracking schedule this weekend but, as soon as I can, I will get you the cwp file. Thanks, again.