Hard bards are going to be a custom fab type deal, but any soft bags should fit. RKA has been a top tier company for a long time, but they're just one of dozens of companies.
As for a backrest, I doubt there's anything pre-made out there currently, but if so it'd be through Corbin. If not, you might be able to contact them about making you a 1-off... though, that'll likely cost more than what I paid for my B4.
It'd be cheaper to buy something like an old Concours that already has those things than to do it to a B4. There's one for $500 in Denver right now that I'm contemplating calling on because it'd be nice to have a tank of a LD touring bike that I can 2-up with my wife.