How much more changeable do you need it to be than to make a new customised copy of it and set that as the default? Do you want to go inside the application package and edit the source file? You’re out of luck there as it looks like the default default is generated on the fly (not from a file), so you’ll have to cope with editing a copy of the factory default workspace instead.
Seriously, how many new workspaces are you making a day where this actually matters?? Even if you could hack the application the resulting user experience would be indistinguishable from using the tools provided, and you’d have to do it for every update. Did you think there was some kind of preference file? There is: it’s called a template, and you can make it the default.