What about fortifying with extra breastmilk fat? When I was skimming milk for DD I had lots of fat left over, and saved it to fortify later. Adding fat adds calories. It is SUPER easy to separate the milk... you could freeze the skim and use it later to make cereal or whatever.
All you need is a big syringe (60cc luer-lock or slip-tip) and an extra pump tube. Cut the pump tube to like 8 inches long, put it on the tip of the syringe. Put the EBM in the fridge and let it separate, like 6-8 hours (sometimes even 12 to get really good separation). Pull the plunger of the syringe back to get about 5 mL of air in it, then put the tip of the tube into the bottle while pushing the air out to keep the fat from getting into the tube. Then just draw up the skim milk from the bottom, and you'll be left with fat. Squirt the skim into another bottle. Sometimes you have to draw up more than once to get it so there's only fat left in the bottle. And it's about a million times easier if you are skimming from a wide-mouthed bottle (I used the Avent VIA containers to skim from). Then you just add the fat to the EBM, and you've got fortified EBM without soy or dairy (or formula at all!)
(Those are the basic directions, if you wanted more detailed directions that I wrote up to give to the hospital to develop their protocol, send me a PM and I can email it to you!)
Oh, and I'd say it's no coincidence that there are issues associated with the formula. The dietician always gave me the line about the proteins being completely broken down, but Faith never tolerated *any* formula (we mainly tried Pregestimil and Elecare).