It would help to have the source nodeId in the "always supplied" parameters for a T Engine transform call. While basing transformations only on content is logical but some transformations might need extra metadata of the source, imagine building a templating engine where some metadata would end up in the specific "template document" transformation.
I cna think of two possibilities, either provide the nodeId to T-Engines, so the custom engine could use it and call back the REPO for extra information, either allow some kind of dynamic transformation options that a RenditionDefinition2 might invoke before checking the transformation option but that is not a good option. I would go with the first approach.
So what is needed, to have a way to get source node informations (not just mimetype) on the T Engine side.
args[i++] = "sourceNodeId";
args[i++] = sourceNodeRef.getId();