Yo Steve,
Just came across your post.
Did you ever get a solution?
I ask because I am just ordering a 35" four string neck for P Bass (well it's going on an old Jazz Bass body) through Musikraft in the US because I hate five strings but love the low B.
You are on the right path with your original musings, the extra inch in scale will give you a better B fundamental. Two options - order a 35" "conversion neck" from someone like Musikraft, which would mean in effect you'd have a P Bass with a slightly brighter treble tone as you don't move the bridge - and the pickups are effectively what move in the scale length,
or order a full 35" scale and move the bridge back half an inch on your P body to get the full 35" scale. Which will still be the beefy P Bass tone but without the silly 5th string and ridiculoussly wide neck to accommodate said silly 5th string.
I'm not sure how the doom guys get the low B sounding good on Rics, probably by using a lighter gauge string, but if you've ever played a 34" Musicman fiver they're pretty loose and muffled when you hit the low B. The extra inch in scale gives a much better fundamental note. Weird bridge bizzo to get a good sounding low B on 34" doesn't seem to make any sense.
There's a reason why so many of the expensive five string basses are 35's.
Anyways