There was a thread about a week ago on the semediawiki-user mailing list, called "Howto install SMW using git?", that became a discussion about a number of things, including why there should be a way to disable extensions installed via Composer. I suggested using one or more global variables for that - like "$wgDisabledExtensions".
There was no real resolution to that thread, so: do any of the SMW developers on this list have any thoughts about this? I think having such a thing is crucial, whether it's done through a global variable or some other method. (Though I can't think of any other method at the moment.)