Even while superbanned, you can update using a cart game that has an update on it, not to mention the upgrade/downgrade app (im not going to try to spell it atm, starts with choidour or something similar) as well. So yes, you can easily upgrade firmware with banned consoles.
I'm in no rush to update to 6.0 when it comes out. In the meantime, I'll just play my games as usual on 5.1. When I do hit a game that requires 6.0 (it will happen eventually), I'll just update with a recent .xci dump. What, ME Worry?