Figured out the issue. My code which called System.Diagnostics.TraceSource.TraceEvent() to broadcast trace events was in another Project. I needed to enable tracing on THAT project via:
<DefineConstants>TRACE</DefineConstants>
I mistakenly assumed that all I needed to focus on was the main target project (i.e. the Web project). It makes sense now - the project/assembly which calls the System.Diagnostics.TraceSource.TraceEvent() must have tracing enabled (via the project properties for the targeted build configuration) to broadcast those traces. Then your trace listeners can hear them. :)