Unfortunately it is not managed, but thanks for the tip in any event.
"If your application consists of more than one module (e.g. an exe and one or several dll's) that use Boost.Log, the library must be built as a shared object. If you have a single executable or a single module that works with Boost.Log, you may build the library as a static library."
Are you sure you did compile Boost.Log itself as a dll too?
In my setup I'm "cheating": I have my own specific logging interface exposed by one module (dll) which internally uses a static versdion of Boost.Log, which is equivalent to haveing Boost.Log as a dll in some ways.