Support of velocity expressions for user-defined properties

Standard properties (artifactId, groupId, ...) are supported in Velocity expressiosn, but still does not work with user defined properties. For example:

<requiredProperty key="a">
<defaultValue>$

.substring(0,1).toUpperCase()</defaultValue>
</requiredProperty>

with artifactId=toto will give a=T, but

<requiredProperty key="b"/>
<requiredProperty key="a">
<defaultValue>$

.substring(0,1).toUpperCase()