The Institute for Advance Legal Studies in London has created a near-equivalent of the Bowman & Harris, Multilateral Treaties: Index and Current Status in a searchable database, although it lacks the multiple source citations that made the print original so invaluable. But it is a great start on older treaties, and free.
On another topic, there is a great deal of talk about nukes and global developments in N. Korea and elsewhere. How timely that the IAEA Nuclear Information Database is now available online, also for free. The scientific literature available in its reports would relate to interdisciplinary research on the treaty regimes and compliance.