Well, I am no blueprint expert, that's why I would take the same approach you took with dynamically adding the JavaDelegates.
A pair of bind/unbind methods and declare the ELResolver as listener like this in a context.xml:
<code>
<reference-list id="activityBehaviours" availability="optional"
interface="org.activiti.engine.impl.pvm.delegate.ActivityBehavior"
activation="eager">
<reference-listener ref="blueprintELResolver"
bind-method="bindBehaviour" unbind-method="unbindBehaviour" />
</reference-list>
</code>
Adding a new behaviours would result in exposing them as a service via another context.xml.