Would it be possibly to make the warehouse inventory slots "lockable"? I.e. to a limit them to a given item? (Just like train cargo wagons or player inventory?)

If a chest can't do it, a warehouse can't do it. Internally they are the same thing, just with different sizes.

So, been looking at the Wiki / Lua-Apiu recently and see that we could technically we can set the inventory type to: "with_filters_and_bar" .. but i'm not sure of the impact of 2000 filterable slots on the engine (assuming storage warehouse)

(yes, they apparently added this ability for normal containers) v1.1.68

An Option here would be to maybe use the https://mods.factorio.com/mod/FilterChests mod to enable filters on the warehouses in the meantime (as they're just containers)

WOW! Thanks for the reply and suggestion! FilterChests does it fact get me to where I wanted.

Obviously it's a different way to use the warehouses but it's fun to experiment with a bunch of surrounding assemblers, picking and taking things from the central warehouse. I did reduce most of the inventory and am just locking rows to various common ingredients :)