I would use sulfites, and perhaps a little potassium sorbate as well, unless you are sure that there are no active yeast remaining in there. In typical meads that finish around pH 3.5, net free SO2 level of 50 ppm will usually inhibit spoilage organisms and with a dose of sorbate will ensure that fermentation will not restart. 50 ppm is equivalent to 50 mg/l, so you need a net of 50 mg of "free" SO2 per liter. However, it is not as simple as just adding that amount. Some of the sulfite will bind with other compounds in the mead, and so will not be available as SO2 to work as a protective agent. It is an OK rule of thumb to assume that the first 50 ppm SO2 of the first sulfite that you add will be bound. So, you want to add a total of 100 ppm if this is your first addition. Given that potassium metabisulfite provides 57% by weight of SO2, so if you are using the potassium powder, then you should add 100/0.57, or 175 mg/l of the powder.
There are sulfite test kits that can measure the amount of total SO2 in a mead or a white wine, but they can be fooled by some of the other compounds in red fruit wines and meads, so I usually just add based on the rule of thumb that I've given you above. NOTE: If your mead's pH is significantly different than 3.5, you will have to make some other adjustments since the amount of free SO2 relates to the total molecular SO2 level (which is the "real" protective form of the chemical) as a function of pH.
Oh, and if I were to backsweeten, I'd use honey. It is a mead, after all!
