As of API version 3.26.10, you can set the marker label with more than one characters. The restriction is lifted.
Try it, it works!
Moreover, using a MarkerLabel object instead of just a string, you can set a number of properties for the appearance, and if using a custom Icon you can set the labelOrigin property to reposition the label.
Source: https://code.google.com/p/gmaps-api-issues/issues/detail?id=8578#c30 (also, you can report any issues regarding this at the above linked thread)