This is the result of a more general problem: the [e]quipment menu doesn't actually let you swap equipment, making it a fairly useless screen (I probably get more use out of it for a quick check of traits than for the equipment itself). With armor, boots, and active weapons, you can use the inventory menu to swap out and it only takes a second: since there is no immediate way to access the prepared slot, it doesn't have this sort of capability and takes almost three times as long (first swap active/prepared, then swap from inventory screen, finally re-swap active/prepared) to do the same task.
Basically, we need a menu when selecting a particular equipment slot that gives the available equipment to swap out, along with an extra selection to "unequip". Doing this is nontrivial: however, I believe it is an important feature that should be included as soon as possible.
Number-swapping doesn't work for missile launcher, so I assume it won't work with other exotics/uniques.
Technically you can command-swap any weapon at all, so long as you bind a key to it in keybindings.lua. This is pretty hacky (not to mention how many exotics and uniques there actually are) which is why I don't use it: ideally we will get rid of this kind of functionality in the future.