Hi Mike,
as per my understanding, there are no standard table to store the tax codes
a standard function module ( HR_GB_TAXCODE_CHECK ) is there to check whether the entered tax is valid or not
i do not think that, we should maintain them manually, look for any SAP notes which has got the tax code changes
if it is an emergency tax code then please raise an OSS message with SAP they might suggest you
Regards
Rahul