we were not notified of his AKLP count until Rocco had already been put to sleep. If we had know we would insisted on a diagnosis before dental cleaning
I agree, this is kind of a loaded question with not enough information. If you don't know why the ALP is so high, you should find out before undergoing elective surgery. It doesn't necessarily mean that the liver has decreased function, but there is obviously something causing either liver damage or some disruption of bone (ALP comes from the bone as well). If you know why (example: the dog is on an NSAID for arthritis), then it may be ok presuming your vet takes proper precautions during anesthesia.
What I cannot understand here is why the vet isn't advising you? Surely the vet is the only person who can know the risks? Presuming it is the same vet who took the Alkaline Phosphatase readings, and is suggesting dental work?
It may be risky, as the high AKLP could indicate problems in the liver (or sometimes Cushing's) But if it is a liver problem -well the liver is responsible for detoxing all medications and anesthetics that come into the body. It may not be able to cope with the anesthetic or sedative required for the dental procedure.
Please do ask your vet about this. Get a diagnosis (what's causing the elevated AKLP) and an idea about whether anesthetics wold be safe -or will be in the future, if the basic condition is treated successfully.