My thanks to all for the input, it's been much appreciated ;-)
I don't build from scratch but modify (or butcher if one is unkind) and revitalise old instruments collected cheaply. I'm using an old French (Egmond) Dreadnought 12 string as the basis here so I've no choice over things like scale length or which end the tuners are sited.
After removing the old rusted machine heads and cracked bridge and sanding the wood back to base, although the frets themselves were OK except for fret zero, somehow it just told me it wanted to be an 8 string acoustic bass ;-)
My solution to the intonation problem is to use a "twin" bridge, as if there was one bridge for the lows and a second for the octaves. Each "bridge" will be crenelated to let the other set pass, although it will actually be one solid piece of wood. "Slant" will be the same for both of course, but I'll have a solid one peice bridge; which is what I'm after. I think these transmit sound better than individually adjustable bridges. It's a compromise I know, but I think it'll sound OK and it'll be more elegant than other solutions.
As for the tailpiece, well, none of the examples for 8-string basses I found on the web had the string order I'm after (a hole that lets bronze Bass Sting through but traps a Bass String ball-end would let a normal size ball end right through!). I'll probably fabricate one out of a decent gauge of brass sheet.