weapprev alias is already there in the game.
I can't give you the config right now (my knowledge gone a bit rusty), but with some alias magic you can do the following:
You simply bind a button to an alias, that on click (+alias) switches to desired weapon, on release (-alias) rebinds the button to "switchalias". "switchalias" rebinds that button to weapprev, and on click rerebinds to original weapon alias.
In quake2 I had a similar one, but for chat messages.
"One should strive to achieve; not sit in bitter regret."