Given that international emissions occur mostly outside of national jurisdictions, the Law of the Sea (UNCLOS) is seen as a firm legal framework for the supra-national approach proposed (derived from articles 89, 136, 137, and 140, among others).
The supra-national approach has also an important decision making benefit. It does not raise questions regarding ear-marking (hypothecation) of a national revenue for specific causes in several countries. Supra-national revenue would always be off national books.
Furthermore, the International Oil Pollution Compensation Funds (IOPC Funds) provide a precedent of direct collection of a levy that bypasses national systems in the maritime sector. The IOPC levy is based on a formula, which is also important from a legal standpoint.
Finally, as the levy would be driven by a market-based formula, and ultimately paid by the importers without discriminating against any exporting countries, it would be compliant with both WTO and GATT rules.