One of the easy ways to solve sandbox issue would be to bind handlers code from corresponding "starter" js files, as their code is not sandboxed. On the other hand, it will probably make it difficult to develop using your test environment, or will it not?
Just realized that Calc included into 2.0.3preview got there from 505/300's (I didn't plan to add it in the preview version for 600 before testing it myself, but it showed itself in menu customizer / key bindings and someone has used it
).
What makes me wonder is why doesn't it show any UI on 600s. Since it works on 300, XML file doesn't contain any errors, so I would expect it to show some UI.