It's not really a "custom Node class" since the DTM is tabular rather than object based... but it is possible to plug in other implementations of the DTM API. See the SQL Extensions for an example, though that's a deliberately incomplete implementation of DTM for storage-efficiency reasons.
We haven't exposed a way to switch the _default_ implementation of the DTM, other than the implicit selection of SAX2DTM versus DOM2DTM based on what kind of Source object you pass into the transformer. (And perhaps XNI2DTM some time soon; I'm working on that.) We didn't anticipate a need to do so. I'd suggest prototyping via the extension approach. If you get something interesting running, we can look at parameterizing the selection of DTMManagers and DTMs.
