B20 Vtec power.

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I don't think that thats the correct way of looking at this though because it isn't just a push pull force but also a rotational force as the rod moves with the crankshaft.

All of which can be represented by simple orthogonal forces.

- and there's never a "straight down" force applied except when the piston is TDC right? The rods are almost always at an angle.

The force from combustion is most certainly straight down onto the piston. From here it splits into two components; one directly down the rod at whichever angle it happens to be at. The other can be computed by vector subtraction, and is the force of the piston against the sleeve.

The top of the rod supports the piston, "added weight", the rod (and ultimately the bolts) are supporting this load whether the piston is going up or down

There are two elements of rod load:
1) Inertia property of the piston/rod. Changing the direction of the mass requires acceleration, which requires force. When the piston is accelerating downwards (upper half of the stroke) the rod is in tension. When the piston is accelerating upwards (lower half of the stroke) the rod is in compression.
2) Combustion forces. Always compressive on the rod and has a the majority of it's power in the upper half of the stroke.

The rod bolt must be able to hold the rod cap on under the extreme tension stress of the rod. When the rod is under net compression, the rod bolts don't have anything to fight - the rod-cap bearing is completely unloaded.

Conclusion: Combustion forces are in the wrong direction to have any effect on the rod bolts.