My project horse last year was very food aggressive as well...I didn't really figure out just how bad he was til I brought him home, where he was boarded (prior to me getting him, he stayed at my parents, and was pastured only). Well after a workout and consecutive cooling out, etc, I brought out his grain...well he about bowled me over...ears pinned, teeth beared, etc...I simultaneously set bucket down, and started bumping his halter, and made him move back...then kept him back about 4 feet...I would bump the halter and flick the lead at his chest if he started invading my space. He eventually grasped that I wanted him to 'stay back'...then I proceded to ask him to come to me; he came with ears pinned...so I sent him back...I just kept on with the 'dance' till he understood that 1) you don't invade my space till I ask 2) when I do invite you into my space, you come in with a friendly expression, NOT with ears pinned and all grouchy.
I eventually started doing the same thing in the pasture; there was a round pen type enclosure where I could 'mimic' feeding him; I would turn him loose, go get some hay, and walk out there, with dressage whip in tow. When he came up all demanding like, I sent him back, smacking his chest with the whip if I had too. I would then proceed to walk toward the middle of the pen...if he crowded me, I would send him away...if he followed me at a respectful distance, I let him be. I would then set the hay down, and stand beside it, and we could continue our 'be nice' exercise. If he came up with a friendly expression, would allow him a couple of mouthfuls. Then I would walk toward his shoulder, and click to him, asking him to move over...not away...just move your body, because I am asking you too. If he got grouchy, I would send him out with my crop and we would start over. When he got to where I could control him around the hay, I would then start haltering him, or slipping a lead over his neck and asking him to leave it, like as if I were taking him out of the pasture where they had a round bale.
Your mare seems to just know how far she can push her food aggression...don't take it, that's all...if she won't let you halter her near a hay pile, then move her away from it, and halter her where there isn't any hay...as long as you are able to control the response, you will be able to get her a little more respectful.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."