I use 8 string and 7 string all the time, and I like to play in different tunings.
A1 E2 A2 D3 G3 B3 E4
A1 D2 A2 D3 G3 A3 D4
A3 E2 A2 D3 G3 B3 E4
A1 D2 E2 A2 D3 G3 B3 E4
my 7th or 8th string not on frets but on the side of fingerboard therefore it is fretless and one could play eastern, arabic, indian etc style of music on it, or it is like a string from normal fretless contrabass, it is my own original idea ...
(c) Andrei Krylov 1994
In reality it is all not necessary for standard guitar music, but since I play mostly my own stuff - I use it a lot.
I experimented with other ways including those two about which you asked and found them unpractical for my purposes.
Good luck in your search!