First off I've only read the first 7 pages of this thread so apologies if what I type here is a repeat of someone else's post.
One way I see this rolling out is to pick class then get queued into a random lobby that's missing that player class, or else get a group of friends/team together and queue to join when a lobby server has space for all. Not sure how the matchmaking will work but happy to let Valve engineers fix that.
'Game' starts and each player picks a weapon they don't want the other team to have. Take it in turns or all at once, doesn't matter. You're informed if your team has already made that selection, and if they have you pick again. Then a 2nd round of voting occurs as before. In lobby this will probably become a meta game as it is with DOTA2. In comp this format will allow the casters to discuss the weapons and the potential strats that come from each one being removed. Gives a chance to introduce a narrative that's currently missing from most shoutcasts. After 2nd round of voting a white list is formed and your GUI with cross out the weapons you can't pick.
It should be set up so that each player will be denied a total of two weapons from their possible load outs. Many weapons considered OP will become staples of the "ban list" but they won't ever be banned before the voting starts.
This will lead to absolutely mad load outs, things that have never been tried before in comp or lobby. Players will learn how to play with and without their trusted weapons and the audience will be enraptured by the myriad of strats and maneuvers that appear.