in jsf you can query beans by name (not by type which is possible e.g. in spring). extval allows to provide a custom implementation of the ValidatorFactory. so you can optimize the lookup for your dependency management container (e.g. to make it typesafe).
anyway, out of the box the bv-integration module of extval (the next milestone of it) allows to provide/override constraint-validators via bean(s).
a detailed description is available at EXTVAL-70.
a spring bean for the example mentioned in EXTVAL-70 would be:
<bean id="customConstraintValidator">class="at.gp.demo.CustomConstraintValidator"scope="prototype"><property name="customService" ref="..."></bean>