With the arrival of IOS 9 it is now possible to switch between the applications through a bluetooth keyboard: (command+tab). There would be a way to remap the Blueboard so that it could send the shortcut commands of a bluetooth keyboard common?
I don't think this is doable. A conversion app would need to be able to 'pretend' to be a bluetooth keyboard and inject keystrokes into iOS from the background. I don't think there is API support for this.
I suspect your only option is to hook up a real bluetooth keyboard to switch between apps.