I'm still thinking
Adding a player to Stereo Tool is really a bit too much - especially with support for all kinds of sources, streams etc. (And people would probably continue to ask for improvements, such as cross-fading, more input formats etc.). A simple .WAV player would be very easy, but might be too limited.
I think it would be a better solution to add a 2nd input channel, and use an external player to provide the input signal. BUT... There are some issues with that as well. For example, if the 1st and 2nd input channel or any of the input channels and the output channel are on different sound cards, they can (and will) get out-of-sync. Virtual Audio Cable would unfortunately also behave as a different sound card (unless BOTH inputs are Virtual Audio Cable inputs).
If you have a sound card with a secondary input channel, using that would work. The setup would be:
1. Sound is taken from input channel #1
2. If there's silence, input channel #2 is mixed in. If input on #1 starts again, #2 is dropped again.
In this situation, you can still use the silence detector that Tekie linked earlier to start a player on the 2nd channel if you don't want it to run continuously. (assuming that you have a 2nd output that you can connect to the 2nd input).
This would be really easy to implement, but as far as I know most sound cards don't have multiple inputs. (If I'm wrong about that, please let me know!)