Only for the insane...

Are you thinking like a standard 8 string with pairs?  If so, you might be able to use a 5 and omit the tuner reams and bridge routing.  I imagine the tuner reams would be tight on the headstock.

If you're thinking 8 like taking a 6 and adding an additional low and high string....whoa...  That would take a custom neck and neck pocket. $$$$ there.  I tried working out a narrow 7 using a 6 but found the strings would come off the end of the neck, even with the narrowest string spacing I could fine. (@ 16.5-ish mm)

Well, I'm thinking regular spacing on the neck as in any regular 4 string, but on a Gecko Body style, one single Music Man styled humbucker, but,

Tune the sucka down to B like a 5 string minus the G.

I'm basically thinking about a 8 string, but in a Gecko Body style rather than a traditional.  I like the Eagle, but that's about the only other body style I like (other than a TFS) than the Gecko's.
How about a G4 body with an 8-string neck? As far as looks, that would get you about 75% there.
That's basically what I'm talking about, a Gecko 8 string baritone bass, B, E, A, D. with the octave for each bass string.