One of my goals during this rewriting of VandalSniper as a more general-purpose browser has been to reduce or eliminate the dependency on platform-specific glue libraries. JSCall# uses a C/C++ library to interact with the DOM, and this is just one more hurdle to be jumped over on the road to portability. Who wants to set up a build environment against Mozilla on Windows? Not me.
To make calls back, I copied an idea from JSCall#. The call is placed by constructing a string containing the function name and argument list, and the document title is set to this string, then immediately set back to what it was. This is picked up by the WebControl.TitleChange event handler, where the pieces are pulled apart into an array, the function name mapped to a delegate, and the delegate invoked with the arguments.
It's one big kludge, but it works remarkably well.