peter,
mysql has two table locking modes: READ, which seems to correspond to postgre EXCLUSIVE lock, and WRITE, which seems to correspond to postgre ACCESS EXCLUSIVE lock. the difference seems to be that in postgre table locking can only be done inside transaction whereas in mysql table can be locked also outside of transaction.
i didn't find in the sources a description of current kamailio db api. lib/srdb1/README claims that it is in doc directory, but i have not found it. perhaps someone more familiar with kamailio db api could check if also table locking could be added to db_mysql module or if the transaction difference is a show stopper.
-- juha