Sounds like you take your scale length cues from the Carl Thompson school of impossible to play basses

I've had five and six strings with 34", 35", and 36" scale. Construction is the biggest factor- you can have a bass with a 36" scale whose B won't hold a candle to a 34" with a stiffer neck. The low B on a 34" will be more taut than any 35" Ibanez B.
I've never wished for anything longer than a 36" scale, and in fact I sold my 36" as the B on my 35" was nearly as tight feeling, went on record with the same clarity, and the instrument was easier to play. Using a thicker string is a much better way of increasing your tension than going to a longer scale length because
A. it may not increase your tension to a noticable degree
B. it may not increase it at all if the construction isn't extremely well done.
C. it will make the entire instrument a bit more difficult to play as the reach between every fret will be lengthened
D. the higher strings sound thinner and thinner as you increase scale length.
Also, a 34" low E has been used on 95% of every recording of electric bass ever made, and no one's complained so far. The best compromise I've seen is the Novax fanned fret system that guys like Dingwall use, although fanned frets aren't for everybody.
As for more strings vs. alternate tuning, I'm sure the guys who order these basses have had those thoughts well before we did. If retuning their current bass fulfilled their needs enough to not need to order multi-thousand dollar instruments, then I'm sure they would have done it. And again, tuning in fifths does not help when it comes to chording, tapping, or even playing the way these guys do. Look out for some videos by Yves Carbonne, Jean Baudin, Garry Goodman, etc. and you'll see what I mean.