Craig welcome to the board you'll find so much information here you won't need to go anywhere else.
Personally I've never had subframe bolts go loose, but I've had plenty of others through the years, as you say a big twin does vibrate a lot and sooner or later you'll notice a bolt missing.
As long as you keep on top of servicing and keep cleaning you pride and joy spend that extra 5 mins doing a quick nut and bolt check and you'll be fine..... got any pics of the bike?
Early closing at the bar on a Sunday night, hope you didn't time this post accordingly
I'll have a Corona though pleeeeez