One severe limitation in Gnudl so far is that few astrophysicists are fluent in scheme. That is why going from a preliminary design to a rapid prototype might be difficult.
Mark Galassi is somewhat fluent in lisp/scheme, and if he decides to start working on the prototype, he will make the effort to learn scheme and guile in great depth.
None of these people have written code yet, but they are participating in or making comments on the strawman design.