I'm currently working on a prototype 8-string conversion on an old short scale Encore P bass.
This may or may not help but here's a few general things about my conversion.
I used banjo tuners as the "extra" tuners, becuse they stick out of the back they take less space. I actually just put four in a line on the bottom half of the headstock and used two of them for bass strings (the tension on the bass D and G is about on the limit of the pegs, they're fine on the octave strings though).
As mine's short scale I was able to use guitar strings for the octave pairs, though to reduce tension and compensate for a standard guitar scale of c26" I used an A string for the E position, D for the A position etc.
I put the bass and octave strings through the same hole on the bridge, let the bass string sit in the usual saddle groove, and let the octave string sit on the saddle seperated from the bass string with a wooden kebab stick!! Like I said - it's a prototype! Intonation on the pairs is surprisingly good.
I have ordered some knurled bridges in order to do this a better but I think a proper 8 string bridge is needed to get perfect intonation.
I haven't given the electronics much thought do I'll be interested in what you do. The Encores P-Bass pickups seem to do a reasonable job across all strings without worrying about the pole positions.
If anything here is of any use to you please feel free to PM me for pictures or anything else. My prototype conversion isn't at all pretty, but more of problem solving exercise.