No, there isn't any default variable for this. A few apps listen to $AUDIODEV, but not all/most of them. So I can think of several options for the few apps with no mechanism to change output device:
A. There are apps which can change their output device, but it's not clearly pointed out in the docs. Can you tell me the name of the emulator?
B. You can change the default /dev/dsp output for the app, since it's just a symlink (i.e. 'ln -sf /dev/oss/.. /dev/dsp'). Once the app has started, you can just change the symlink back. A script can automate this (A more permament change of default output
would require a bit more setup due to vmix considerations).
B. 2. A variant of the above - a simple lib can intercept the open request and change /dev/dsp to something else.
C. You can recompile the app, and change "/dev/dsp" string to something more useful.
D. If the app can output via Pulse/libao/SDL etc. these libs/sound servers have a mechanism to use a different output.