9G
In my view, you dispatch according to the MEL/CDL, if there are stipulations about the STBY ATT/STBY ALT in the MEL/CDL, then you comply.
If, in flight, with everything serviceable at dispatch, then the QRH governs.
Hence, if at dispatch, there is no restriction on the STBY ALT, and the QRH does not require it for Cat 2/3 (as is the case for A320/B747-400, to my knowledge), the even with a failure/flag in the STBY ALT system, you can perform a Cat 2/3 approach and landing.