Java datatype library: value object->string conversion

I'm working on a Relax NG implementation which needs to convert data 
values two-way, both string->object and object->string. I'd like to use 
the datatype library interface for this purpose, but I can't seem to 
find a method complementing DataType.createValue(), one that converts a 
value object to a string. Is there a reason for the API's read-only 
design, or am I missing something?

Julian