The batch I made for Burning Man came out great. I used welch's Unfiltered Grape Juice which took a couple months to clear, then backsweetened it with more honey and grape juice which finished around 1.030 with a lot of Welch's flavor still. From there I carbonated and turned it into Grape Soda for grown-ups.
It was really popular at burning Man, and I would just roll out on my beer cart and post up on a street somewhere and make people stop and try some. I still have about 2 gallons left at the house, and it will be my go to for people who are not beer drinkers. Will probably keep it on tap permanently, but may change it up a tad. I think I liked the regular grape juice over the unfiltered, but I may have just backsweetened this batch too much. It works though, its like grape soda that is 12-15% alcohol or somewhere around there. Never took an OG since the honey just fell to the bottom.
In regards to the question above, you can do that, but its a huge amount of headspace, just make sure to get it out of there once primary fermentation is done. This recipe can sit and age for a while if you want it to, but not with that much head space.