No I haven't ridden either of those boards. But like ETM says, they're is plenty of off-piste action. There has been so much snow this winter in Hokkaido that you've got very good odds of getting thigh to knee deep powder (literally) if you duck a rope at the right resort and you know where to go. It's only in the last 10 days that I've actually had difficulty finding untracked pow in and around the resorts.
I don't know a whole lot about the Billy Goat, but I think the Hovercraft would be great fun to ride around the glades, and maybe just above the treeline. The board looks like it's got a lot of float and to be very nimble. I'm riding a T.Rice 156 which is awesome to ride on any piece of terrain I've taken it through, but it really feels the best above the treeline on steeper runs.
If you can afford it I would seriously look into a split-board. Unless you've got a snowmobile, snowcat, or helicopter you'll be using snowshoes every time you go BC. With the split you won't need to lug around snowshoes, and you can use skins to make your ascent. On the downside the bindings for splits are usually more expensive, and skins aren't cheap either. Furthermore if everyone in your group is wearing snowshoes it may be more beneficial to hike straight up the mountain rather than doing switchbacks which is more common with skins.
Personally I've never ridden one, but I would love to have a go, and am probably looking at getting a split for my next deck. Just think about what kind of terrain you'll be taking your board through before you buy it. I'm really happy with my deck which I bought when I was living a couple of hours out of Tokyo last year. If I knew I'd be moving up to Hokkaido and doing BC fairly regularly I might have gone with a split.
Before getting into the BC I would recommend riding through wherever the powder is at a hill near you, maybe you have some local mates that might show you some powder stashes. If you do come up to Hokkaido there's plenty of powder off piste in Rusutsu, although not any really big lines. You can also go through the gates to some outbound runs in Niseko and it's a bit more open than Rusutsu. There's also a gondola that takes people about 3/4 of the way up Asahidake (biggest mountain in Hokkaido) which is great for powder runs, and longer lines than you'll get at Niseko or Rusutsu.
After you can ride the slack country powder is when you want to take a look at some BC tours. There's a couple of crews up in Niseko that run guided tours, and one of my mates in Sapporo does as well.