Ok I have already done it. I got the lamp from a crashed S40. I am not sure if it is the same as the S70's. I have not needed those holes that you refer (I am not sure that their porpouse were a "future center high-mounted brake lamp").

I have not needed screws or glue, simply the lamp which is quite slim fits between the ceiling and the celing garnish, pushing it with a light presure enought to not have clearance, it will not move. It works and fits fine.

It is not the most polite method I kwnow, but you will not notice it looking the car from behind.

There is another option, there is a manufacturer of electrical automotive components and parts (RINDER) that make many models of brake lights with nice and avanced designs. There is a nice model (ref. 777) in which the lamp is fitted to the glass by means of adhesive (without supports or screws). You can mount it easily in the upper or lower glass liner. It is quite cheaper than an original Volvo lamp and with a very nice design too, and much more than the original 850 single lamp. About 15€ ($15 aprox.). I recommend it.

I hope have helped you.

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850 GLT Aut.