Hi Markus,
unfortunately 6.1 only applies if the subgenus is part of a bi/trinonmial.
Article 6.1 says that "the scientific name of a subgenus ... must be interpolated in parentheses ..." which implies that what between parentheses of ariticle 6.1 style name is the scientific name of a subgenus. See Aritcle 4.1 also where names higher than species group are requied to be uninomen.
If the name itself IS the subgenus it is apparenlty valid. For sure its common practice even for recently published names: http://zoobank.org/NomenclaturalActs/45E20811-FEC5-4DBF-B7CB-D39AC32A8BFF
ICZN glossary says as
interpolated name A name placed within parentheses (1) after a generic name to denote a subgenus, (2) after a genus-group name to denote an aggregate of species, or (3) after a specific name to denote an aggregate of subspecies [Art. 6]. Names used in this way are not counted as one of the names in a binomen or trinomen.
I interprete it as Amphiglossus (Brygooscincus) denotes the subgenus Brygooscincus where Brygooscincus is the subgenereic name, It is a common practice, rather than a secientific name.
On page 66 of the publication http://www.smuggled.com/AJHI28-29.pdf which is the source of given ZooBank record uses "SUBGENUS BRYGOOSCINCUS SUBGEN. NOV." on the section title, and the description starts with "The subgenus of Amphiglossus Brygooscincus subgen. nov." The author uses subgenereic name Brygooscincus in parentheses only as species name.
Note that the ZooBank recored's spelling attribute is Brygooscincus.
James