@XStreamImplicit only supports fields of type Collection but there does not seem to be any compelling reason for why this behaviour shouldn't be extended to fields of type Map.
Developer mailing list conversation extract:
BB> 1. @XStreamImplicit doesn't work on Map fields.
GS>>I dont know what was the decision at that point, but we could surely make
GS>> it work with Maps, have you seen Annotations.java sourcecode? We can make
GS>> it somehow available.