The answer was this:
SELECT * FROM MSStorageDriver_ATAPISmartData WHERE InstanceName='[PNPDeviceID]'
Just make sure to double-escape any backslashses. So if the PNPDeviceID as found in Win32_DiskDrive was
IDE\DISKHITACHI_HDT725050VLA360_________________V56OA7EA\5&276E2DE5&0&1.0.0
what would be returned by getting the value will be
IDE\\DISKHITACHI_HDT725050VLA360_________________V56OA7EA\\5&276E2DE5&0&1.0.0
but what you need to send in the WHERE clause is
IDE\\\\DISKHITACHI_HDT725050VLA360_________________V56OA7EA\\\\5&276E2DE5&0&1.0.0
Silly, isn't it?
Oh, and from what I've gathered, you also need _0 on the end of the Device ID, so all together, you would send:
SELECT * FROM MSStorageDriver_ATAPISmartData WHERE InstanceName='IDE\\\\DISKHITACHI_HDT725050VLA360_________________V56OA7EA\\\\5&276E2DE5&0&1.0.0_0'