It is not possible in the BF2 engine, at least not without also allowing people to "deploy" their weapon on thin air, like in the middle off the street. So it's a compromise.

1. But,I think you can try to set the attribution of the machine-guns as same as the ordinary rifles,I mean because the common rifles have a function called "Targeting mode" which can be activated when player right-click their mouse,analogously,maybe you can add "Right-Click:Activate Targeting mode" function to machine-guns,and link up with the motion of "deploy" machine-guns.In other words,maybe this function could be realized like this way:Player can Right-Click and see the "deploy" motion of machine-guns,but computers still think this "Right-Click" is a common rifles' "Targeting mode" command.
 2. I have to remind you because you seemed like misunderstood me:I know in WW2 history,machine-guns shouldn't be deployed at thin air or in the middle of the street and I didn't mean to realize that at all,but what I wish to realize is deploy MGs at historical right positions,like:prone position(which already be realized),behind building's windows,sandbags,some laigh breast walls or any other applicable blindages.
 3.As we all know,BattleField 2's game-engine is actually too old and the technology of game industry is developing more and more maturer now.Will the Forgotten Hope team transfer and work for next generation of BattleField? Tell the truth,I think it's a great pity that when Graphic Technology wasn't mature,worldwide famous game developers’ interests was focusing on the World War 2;but when today’s Graphic Technology’s revolution has reached photorealistic level which I regard as a milestone revolution,the mainstream public opinion of game industry think World War 2 is an outdated theme and the WW2 theme games market had reached its saturation long ago at 2003-2005.So,today’s game industry is lacking of some WW2 theme games with the best uptodate Graphic Technology.(By the way,my favourite game type is the game which combine with RTS,RPG&FPS.But only few games realized this.e.g: “Heroes&Generals”)I don’t like magic type of game,because the earth has how many people on the world,there would be how many types and criteria of magic.But the real truth of the history is only one,which means the relevant criterion of historical theme is only one and WW2 is a such example.I mean if the game industry has no ability to create a perfect realism or historical theme game,then just develop magic games,I think those game couldn’t be as good as they assert.