I came up with this one while working on the 550 Master cylinder. There's a lot more room to work inside if the plunger is pressed down. This allows the internal snapring to be removed with average snapring pliers, rather than the specialty ones sold by Honda, Motion-Pro, etc.
With all of the fluid drained out of the master cylinder, remove the brake lever. Thread the lever pivot bolt back into the MC. While holding the MC with one hand, insert a screwdriver so that the tip presses on the plunger while using the lever pivot as a fulcrum. Pull the handle of the screwdriver up to depress the plunger. The snapring can then be removed.