I think that the "simplicity" of a mechanical system in this case will be a lot more complicated to achieve than using servos, particularly if you've got dihedral. In fact, if I were tasked to do this I'd be tempted to drive a pot with the throttle arm of the bellcrank, and put servoes in each nacell.
At any rate -- it's going to be different for each plane. I'd probably use little ball links on 90-degree bellcranks to actuate the throttles from rods moving span-wise in the wings. I'd probably have a master 90-degree bellcrank in the center section (assuming dihedral) to translate motion from the bellcrank to the throttles. I may decide halfway through that ball links have too much friction and need to be replaced by z-bends.
You're going to have huge mechanical fit and friction issues. If it were me I'd probably dummy up the whole control system on a slab of plywood, leadouts to motors, with all the appropriate dihedral breaks and all the engines in their final positions. I would do this as the very first step in actual construction. I would make it work perfectly, knowing that whatever goes into the plane won't be any better than what I could achieve in such ideal circumstances. I wouldn't start on the plane construction until I'd satisfied myself that the control system worked well enough, had low enough friction, didn't have leadouts rubbing on any throttle-control bits, and had room to fit in the actual wing. Then I'd build the wing first, referring back to the control system as I went. There is a good chance that I'd transfer the actual bits of the control system from plywood to plane. I'd make everything adjustable. I think I'd also see if there were any scale or scale-like access panels that I could put in over all the critical points so that I could maintain the thing, so I didn't have to cut into my nice scale plane halfway into the first competition season.
Then, before I started work, I'd think again about the convenience of just bunging in a receiver, battery, four servoes, and some servo-wire tunnels...