We could easily support both xml and json for solver configs, by adding SolverFactory.createFromJson*() methods:
https://github.com/kiegroup/optaplanner/blob/master/optaplanner-core/src/main/java/org/optaplanner/core/api/solver/SolverFactory.java#L88

XStream supports JSON too, so it's just a matter of copy XStreamXmlSolverFactory and using a different XStream setup:
https://github.com/kiegroup/optaplanner/blob/master/optaplanner-core/src/main/java/org/optaplanner/core/impl/solver/XStreamXmlSolverFactory.java#L50

We'll probably want to rename XStreamXmlSolverFactory to XStreamSolverFactory and then make the json/xml switch a boolean constructor parameter?