Bug description

I see in the documentation that I can use vspacecode.bindingOverrides to customize existing bindings, or add new ones in a menu. It works great for "simple" bindings, but I can't manage to find the right syntax to customize a major mode-specific binding.

Example: I want to change the command used to toggle text to bold, only in markdown major mode.
I want all the other markdown-specific bindings to stay like they are, I just want to modify m x b for example (only for markdown, that is)
Is it possible?