Saxon

At some stage in the past it was possible to call NodeInfo.setSystemId() to set the system ID of a node. The method still exists, because it is needed for NodeInfo to implement the JAXP Source interface, but it will generally throw an IllegalArgumentException; this is because the data structures used to hold system ID and base URI generally don't allow these properties to be changed for one node with complex side-effects on the properties of other nodes.

It would be useful (and easy) to provide a virtual tree model (analogous to SpaceStrippedTree and TypeStrippedTree) that essentially acts as a virtual copy of an existing tree, but with remapped base URIs and or system IDs.