Caused by: java.lang.ClassCastException: org.jboss.deployers.structure.spi.helpers.AbstractDeploymentUnit cannot be cast to org.jboss.deployers.vfs.spi.structure.VFSDeploymentUnit
at org.jboss.deployment.security.AbstractSecurityDeployer.deploy(AbstractSecurityDeployer.java:148)
at org.jboss.deployers.plugins.deployers.DeployerWrapper.deploy(DeployerWrapper.java:171)

In AbstractSecurityDeployer this code

// if policy registration has been set, check the deployments for XACML and ACL configuration files.
if (this.policyRegistration != null)
{
VFSDeploymentUnit deploymentUnit = (VFSDeploymentUnit) unit;

assumes that you pass in a VFSDeploymentUnit

... it might mean you cannot deploy WebMetaData dynamically in AS50