I've used the Ritchey torque key before. And then used a regular allen wrench. It seemed to me that by using a normal length allen wrench and tightening it to the point where there is resistance but you're not straining at all, I could achieve pretty darn close to the 5nm of the torque key.
I don't think tightening this Al/C seatpost is a big deal. Just use a friction paste and tighten with an allen key at a reasonable level.