I think that would be nice to have the possibility to change the scopes & barrel attachment in multiplayer just like you do in single player.
Also it could be done just like in single player: you just have to hold the select/back button and then toggle between attachments using the standard buttons; of course just pressing the back/select button would open the scoreboard as usual.
I really think that this could add a true new level of tactical possibilities and also is very useful if you have unlocked many attachments.
Anyway the possibility to change attachment would be strictly limited to the scopes & barrel attachment so if you wanted to change the "under barrel attachment" you would have to wait to the next round or go to the Change Equipment menu!
What do you think about it guys?
P.S.
I don't have Crysis 2 for PC so I have explained my idea from a console point of view but surely this could be applied to the PC version as well...I simply don't know the controls scheme for PC.