What is the meaning of "on an exceptional basis"?
For this purpose, a close enough definition is in special circumstances.
It is saying that a manufacturer can only use a listed substance if given an exception from the rule. And exceptions would only be given in special circumstances. The opposite is “on a routine basis” which means something that is done often and without special circumstances. An exception from a rule can be given on a routine basis, but in your paragraph they are saying that it would be allowed by the committee only on an exceptional basis.