Oh dear. With nothing to center the hole saw, you'll have a lot of excitement. The only way it could work would be if you'd make plugs of aluminum to fill the holes, and clamp the brace tightly on them. Then the center drill of the hole saw could drill into the plugs. But there would still be a fair amount of chatter, to put it mildly.
A vertical mill and boring head would make it a relatively easy job. If you could find a small local machine shop, you might get a reasomable quote.
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