The basis would be the withdrawal treaty (as well as general international law) as set out above. (eulawanalysis.blogspot.ru)
This is because Belgium argues that Swiss Courts are under an obligation to stay proceedings brought in that country not only as a result of the Lugano Convention but also because this result is dictated by the rules of general international law relating to jurisdiction. (ejiltalk.org)
Furthermore, as Matthew Happold pointed out in a previous post, general international law does not provide for diplomatic asylum. (blog.politics.ox.ac.uk)