I'm afraid using the trunk can sometimes be 'dangerous', I've just refactored quite a bit and the online documentation isn't valid anymore. (Mind you, it says version 0.8, so you can't blame me haha).
The stagewiring logic has been refactored into a seperate interface called IStageProcessor, which will allow other types of stage processing besides autowiring. (I've just committed a localization package yesterday which can assign resources to stage components, for instance).
The override has slightly changed for the autowiring at the moment, right now to override you'd need this markup:
Mind you, this might remain subject to change
<object id="flexStageObjectSelector" class="com.beetlemed.webapp.spring.AutowireSelector"/>
<object id="flexStageObjectDefinitionResolver" class="org.springextensions.actionscript.ioc.wire.DefaultObjectDefinitionResolver"/>
<object class="org.springextensions.actionscript.stage.DefaultAutowiringStageProcessor" id="defaultAutowiringStageProcessor">
<property name="selector" ref="flexStageObjectSelector"/>
<property name="objectDefinitionResolver" ref="flexStageObjectDefinitionResolver"/>
let me know if this works for you.