This may be a little late but you might consider that the intent of 250.66 is to provide a grounding electrode conductor that has a cir mil area based upon the cma of the supply conductors. In the case described the busbar would be considered the service entrance conductors???
The conversion from area in mils sq to cir mil area is Amil sq divided by .7854.
A 1/4" X 2" bar would have a sq in area in mils of 250 X 2000 or 500,000 sq mils.
divide 500,000 by .7854 to obtain 636,618 + or - cir mil area.
reference to 250.66 and select a 2/0 CU or a 3/0 Al GEC .
Although one could consider busbar as not fulfilling the intent of the term conductor it is conducting material.
just a thought!