I am afraid that at this point in time, the design and theoretical issues can not be easily broken down in small tasks for external contributors.
It is certainly possible for an external contributor to publish research articles on the subject, write down full technical RFC, or work on becoming an OCaml type system expert (for instance by cleaning up the existing typechecker code).
However, with this level of involvement, it is quite probable that you will no longer be a external contributor by the time you make contributions on modular implicits.