I use big huge fat 13 gauge strings... that's why. Even though it's at Drop C, I've got string tension to spare.
I haven't attempted to drop it to "A". Maybe it's possible in theory... what I would do is put the E string where the A string is supposed to go, the A where the D is supposed to go, etc. The problematic part would be getting the quote-unquote E-string down to "A."
On my 5-string bass if you drop the B-string below what it's supposed to be at, it'll turn into a floppy useless bungee cord. (I tried it once.) I imagine a baritone guitar would be the same way.
However they do have 7 string and 9 string basses where there's an F# string below the B-string... and a 9-stringer will have one below even that, I think... which would be... (counts on fingers)... uhhhhh... C#... whereas the high skinny string on a 6 stringer (and as well a 7 and 9 stringer) is a C.
Bill "The Budda" Dickens is known for doing his thing on a 7 stringer and sometimes a 9 stringer. However, he usually has this big fat rope thingy that serves as a capo that he straps around the fourth or fifth fret. See, it gets to the point that it's no longer audible to normal humans... unless you have ears like a humpback whale or something.
I know of someone who constructed a 4-string bass that had F#, B, E, and A. He must have had to build the thing from scratch... or he did something to seriously tweak the neck and bridge.
Dickens likes having that many strings because he can go from low to very high very quickly and mind-blowingly intricately by working up and down the strings rather than having to go down the neck to the higher frets and back up again to the low frets. Do a web search for Dickens' website and you'll be able to see videos of him in action... frightening to watch. He pulls stuff that virtuoso guitarists can't do on their guitars.
Left of Center mp3
Open the above webpage and download the "Left Of Center" mp3 in the upper righthand corner... can't miss it. It's Bill Dickens doing his thing on a 7-stringer.
Now... for your guitar... one thing you could attempt, at a very possible risk of wrecking your guitar, is to take the D-string off a bass... you might be able to shave the nut and bridge to accept it... but chances are you will seriously damage, if not destroy your guitar... and for what?
