Hi,
SET CONSTRAINT is not DDL, it is a Transaction Control Statement, as seen in Oracle docs.
Take a look at
http://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_1001.htm#i2099286.
It does not imply a commit, so there is no need for an autonomous transaction.
Anyway, you still need the "EXECUTE IMMEDIATE" construction.