Right, you just have to start with an instrument of appropriate neck dimensions. I'm not sure what nut width that would require, but you could almost certainly convert a five into an eight. If you start out with something a little wide, then a neck shaving might be a possibility as well.
As far as the tailpiece goes, you could probably file new slots for the smaller strings. Plated hardware would complicate this, but straight brass should be pretty simple. I don't think you'd intonate correctly if you tried to just cut additional slots in the bridge saddles of a four string bridge, though, so you'd have to spring for that. Finally, don't forget that there will be some neck/headstock finish work to be done as well.
I'm sure it won't be cheap, but converting a $1500 Epic or Spoiler/Persuader might be cheaper than the $4K you might have to pay for such a bass on the used market today, not to mention a lot quicker to get your hands on. No idea at all what the least expensive price would be for the simplest eight string built from scratch.
-Bob