Hi guys, long time reader, first time poster.
I know it's not quite the same, as the 12 string will have greater tension on the neck, but Bill Conklin puts a high A string on his extended range guitars, he uses an 8 gauge string, I believe. It's only a tone below what you'd be trying to achieve which leaves you the option of seeing how you like the extra tension (i'm guessing it'll be pretty high) or tuning your instrument A-A, rather than B-B (which I've heard alot of bari players do).
I've never picked up a Conklin so I have no clue what the high A feels like to play, so I guess it'd be helpfull if any Conklin players on the board chipped in their views, but I just thought I'd give my $0.02
Nate