We're planning on using a Cabot semitransparent exterior stain, mainly because it's what the lumberyard carries (I have an account and 10% discount there).

The plan is to create two troughs (or a gutter and a trough), the gutter will either be a simple open ended V created by nailing two planks together or it will be cast off rain gutter.

The trough will have sloped sides, solid ends and a flat bottom ~1" wider than the siding boards. I'll line line it with heavy poly, set it level and fill it with a few inches of stain, I'll set a couple of cords across it to facilitate removing the boards.

The gutter will sit below and slightly off line to a length of clothes line strung between two trees. It will be elevated to slope down slightly to a bucket which will catch run off stain.

The idea is to drop the boards (gently) one at a time into the trough and submerge them in the stain for roughly a minute so they soak up as much stain as possible (treat them to rejection). Lift them out at an angle so they shed excess stain and then stand them on end in the gutter leaning (back side or belly) against the clothes line until they have ceased to drip. I'll string additional lines about that they can lean against to finish drying before stacking.

Rubber gloves and ratty old clothing will be the uniform of the day.