I purchased Pinhead security skewers, and the instructions suggest to tighten them to 8 N-m (70 in-lb) of torque. However, the skewers use a special key which doesn't appear compatible with a torque wrench, so how can I figure out how much torque I am putting on them?
Put your torque wrench on something else to get a feel for how much force you need to apply to the tool you have, then do your best. My cynical take is that anyone who can't make their gizmo compatible with a torque wrench has no right to expect accurate torque.
Looking at their website the key does seem to have a square-drive option if you take it apart. I screen-shotted their zoomed view to get this:
If you're lucky that axle will be removable, either because it's got an allen head and can be unscrewed, or it pushes out. It might be a destructive removal so maybe do that to a spare key.
Once you have the square-ish head exposed I expect you'll be able to find a socket that fits it for your torque wrench. Those 12-slot sockets are designed to take square or hex bolt heads, and this is one of the few times with a modern bolt that you'll actually use that feature :)
I'd be doing that anyway because that key is just obnoxiously large to carry everywhere (which you have to do if you want to be able to put on a new tube).
I only recently bought a torque wrench small enough for bike use. You can get quite a good idea of the torque by working through the physics to find what mass you'd be lifting given the leverage of the tool, then applying a similar force to lifting such a mass (ideally something you'd hold in a similar way). For pinhead skewers this works out to being quite firm with it but not using maximum effort. For me.
Torque measurements don't say very much about the actual tension force in the skewer anyway, because the relationship between the two depends on the friction between the nut and the skewer,as well as between the nut and the drop-out.
I could be wrong but I've a feeling they sell a socket type adaptor for the skewer type kit as well as the keyring.