Taking items has the shift modifier to only take one, would be very useful if there was a shift modifier to put only one in.

InventoryTweaks lets you split one item from one stack into another (and into a container if possible) by rolling the mousewheel.

What about shift-mousewheel-up/down while pointing at a drawer? Some other mods use shift-mousewheel only for specific multifunction tools... I can't confidently name one, but I think EnderIO's Yeta Wrench does that.

Shift+Right already has special meaning in Minecraft to place a block against a block with right-click activation.

Completely forgot about that. I've only been putting items into them so far, so I hadn't ran into that yet.
Maybe some other modifier could be used?

Is there another key that has a precedent of use that doesn't require a custom keybinding? Keybindings in modded minecraft are a huge mess.

What about Shift+Right using a bare hand?

Already assigned to pull up the upgrade management UI.

OK. What about Shift + Right with a bare hand while standing in water on an even numbered day in a plains biome? 😀