I believe it has happened twice:

1. Volvo 142 valve springs and

2. Rear hub/disc conversion on the GM Quad 4 cars.

The others as Greg mentioned were shot down. [/b]
If the Volvo valve spring allowance ever happened, it must've been many years ago. There is no such current allowance, nor has there been for at least the past 4 years. The GM rear hub allowance however, is currently in the ITCS spec table "Notes:" column.