I’m staying at pop right now. There aren’t any buses to magic kingdom until one hour before park open, so today that’s 8am. Like you, I have taken buses to early breakfasts about 100 times before. But the transportation CM on the ground at pop at 6:40 said it couldn’t be done; I’d have to Uber. I took a risk and bussed to animal kingdom (8am open, so first bus left pop at 6:50) then asked a bus driver there to help me find a bus to get to breakfast at contemporary. He said he would call dispatch after he inspected his bus. He ended up being the bus to take me to the contemporary. Since I had to wait for him to finish his inspection, I was 2 minutes late for my 7:40 ressie.
But I think I got lucky with a nice driver at the animal kingdom. And trail’s end is trickier because it’s by the boat dock not the bus stop. I don’t think boats would be running that early because they just don’t want guests in the Magic Kingdom area more than an hour before the park opens.