I have reviewed the document and it is generally free of security considerations as claimed. There are some areas of concern however, the significance of which may become more apparent as such tools find future use.

As described in the document, the tree diagram format is intended to serve as an output generated by a tool to aid human interpretation. Thus, a potential ambiguity can arise if the tool used to generate the format is buggy or if the document contains schema and presentation data compiled from different versions of the source. 

Specifications using this representation need to make clear which representation is canonical. Otherwise we end up in a situation in which a document that has an ambiguity being unfixable by means of issuing an errata because there is no agreement as to whether the change is breaking or not.

Another issue that is of concern is that even though the format is not intended to be an input format, there can be no guarantee it will not be used as such. Indeed it could be argued that a spec that makes use of this format should encourage this approach so as to detect possible ambiguities.