Yeah, I've had a play around with an Inventory Turtle and I can't see a way to get it to re-stock an EnderChest from an ME Interface. The only thing that I know of that can inspect inventories and act on the contents of specific slots is the Sortron. You'd need two Sortrons, one to step through the Enderchest slots, and the other to step through the ME Interface slots and pull the right numer of items to re-stock it. Good old RedPower!
Hm, maybe you could do it with a turtle bucket brigade. First turtle is above the EnderChest. It pulls a stack, and compares it to the eight items in its inventory, and if it doesn't match, it passes it forward to the next turtle. If it does match, then it pulls the appropriate number of items from the ME Interface above it to fill the stack. You'd need an Inventory Turtle to do the pulling of the matching item. The next turtle, when it receives an item from the turtle behind it, does the same, it would need another EnderChest underneath it. Each turtle would have the same 8 items in its inventory as the ME Interface above it is programmed to keep stocked. When there are no more items to pull, it sends a redstone signal forwards which all the other turtles then propagate forwards, and they all dump their 8 items down into the EnderChest. So every now and then you'd see the pouch emptied and shortly after it would be refilled. Three turtles could keep a pouch stocked with 24 different item types this way, and a fourth could top it up to 27. Hm, you would have to limit it to 7 item types I think, otherwise the first turtle might fill its inventory with all 8 types (plus the 8 comparison items) and be unable to pass on to the next one.