My PhD is a methodology for assessment level of ethics at Cuban organisations, many procedures evaluate sustainability, or CSR but I haven't found any method for assessment ethics business.
Basic tenets of software engineering are related to seeking ways to address a complex situation through various approaches/tools and models. Two of these are perhaps requirement engineering and measurement. These two in fact, if pursued properly, could provide the way to look forward. One need not worry if any supporting approach is available or not.
Juan, you might want to look at the U.S. Foreign Corrupt Practice Act (FCPA), as since 1977 has been enforced across all OECD countries (to some extent or another). Although it covers mainly bribing/extortion of government officials, it provides a good framework for developing ethical behavior across borders.
Have a look at Professor Ralf's Müller papers and books on governance, trust and ethics. He is an expert in this field and you can also contact him directly on researchgate for his advice.
It is better to narrow down your focus --- sustainability in what sense? Managerial level? Program level? Etc.? --- it is because, in doing so, you can identify an ethical issue that is more manageable to work on. Also , there are global forces that shape sustainability practices of corporations into CSR (you may want to initiatives of the concerned UN agencies; ISO) via their suggested assessment protocols --- corporate practices respond to these forces.