A command or some such to raise a "hand" and arm to block the sun. (edit: Maybe Shift H) 
This is a really neat imaginative idea that would be would cool if it could be done, but...
Can you imagine how difficult it would be to code? Where to you put the hand each time? Would the hand move to stay in front of the sun as you maneuvered your plane, or would it be in a static position? How could you throw the hand up in a different position each time based on where the sun is in relation to your view?
Seems to me it would be very challenging to implement, (would it?) and in the end, it would be a great deal of effort for a very minor change in the AH game overall.
still thinking... I suppose, you would always know the position of the sun, and the position of the head. The hand could be distance X from the head on a vector from the head to the sun. That would keep it in the right position while you maneuvered around... Well, that would be a little better than RL, now, wouldn't it?
Maybe instead of a hand, you could just put on your sunglasses with a toggle.

Ahh, Just enjoy the sun glare.
