I have no experience on this, but I have given it some thought. Here are some of my ideas.
First, the bridge. I believe the best way would be to use one-string bridgesone-string bridges, like M_A_T_T used on his bass pictorial. They would allow any string configuration with any kind of spacing. Another one would be to have a custom 10-string bridge (or however many strings you want) made, or make one yourself. I believe going with multiple normal guitar/bass bridges would only bring trouble.
The ideal solution for the pickup problem would again be to make your own pickup, but I know that's not everyone's cup of tea. Another option would be, like you said, to combine multiple pickups in a P-bass kind of way. I would probably go with EMGs, but any kind of soapbar would probably do.
Does anyone have any experience on how a bass pickup sounds with guitar strings?
The strings... Warr Guitars (maybe the others too) use locking tuners, which allow using much shorter strings - you only need enough length for the string to reach the tuner, plus maybe an inch to spare...
One thing I have been thinking about are the frets. Would some special kind of frets be needed/advantageous? What kind of fret wire would you use?