These restrictions on overboring are really silly (not just SCCA).
Allow a 1st overbore. Most engines these days are metric, and they don't come in .25mm increments, they come in .5mm increments.
If you are concerned with performance increases, specify that overboring .020" adds XX pounds
The cars are so heavy now that I think 50 lbs. would be fine
Also keep in mind that the larger the displacement, the less horsepower the restricted cars will make (these are currently the class leaders and the ones to be concerned about) - yes a little more torque, but not enough for a competitive advantage.
You should also specify that if 1st overbore pistons & rings are used, they must be Factory OE pistons & rings - no aftermarket pistons allowed.