The title pretty much says it. Is there some sort of event for this? I've seen it done by other mods before, so I know it's possible.

I am on 1.8 so no help here, BUT - notice that blocks like Workbench only 'work' when you are not sneaking. (sneaking makes you place blocks).

Use reverse-tactic (look vanilla).

 

Also - if you don't want to use player.isSneaking (or whatever the field's name is)and need that value on server, you will need to make IExtendedEntityProperties and store when the button is pressed and when not (send packet from client to server - on click "true" on unclick "false").

 
public boolean onBlockActivated(World world, int x, int y, int z, EntityPlayer player, int i, float f, float g, float t){
if (!world.isRemote && player.isSneaking()){
            //Player is Sneak + Right Clicking
        } else {
            //Player is just clicking
        }
        return true;
}

Thanks guys, I'm away from my code atm, I'll try that tomorrow...