I understand the appeal of a supported JSON report. It's probably a good idea. But it should be a separate command, not a --json switch on the text report. XML and HTML are their own commands, not switches, so JSON should follow suit.
are you OK with the changes to summary.py + a new json command ? Adding new command is relatively easy but I don't think it is a good idea to repeat the report method of SummaryReporter or create a whole new class (like JSON reporter) for that ? What is you preference ?
I would think that the amount of code that's different between "report" and "json" would be much larger than the amount of code that is the same between them. I would prefer a new class to handle this report.