Normally, Output 2 is used for outdoor speakers/sirens. It can be configured for either voltage or audio output via Global definition G26. If you set Output 2 to Voltage, then sure, you can connect a screamer to it. It can drive up to a 1 Amp load.
If you want to limit the conditions under which the screamer will sound (e.g. in Armed Away mode, but not in Armed Stay), you'll need to use a relay that is controlled by a rule. Out 3 could be used, or a separate relay, like a 912 or 924, driven by a voltage output. As long as you have a relay in there, then you could just as easily power the screamer from VAUX.
