The Bane and Crab pieces are the same pieces. The only difference is that Crabs have the back piece with the legs. The other costume bits are all the same.
If you look at the NPCs you can see that they are the same too, just on different models and using different coloring schemes.
Ah-HA! So I WAS being obtuse!
Thank you! 
Aside from the back details, Crabs use the normal, grey-ish normal whereas you need to use Custom for proper Bane coloration. I also think Bane Spiders use thinner leg armor, but I'm not 100% certain on that.
Either way, I recently respecced my Crab into a Bane, and his uniform is virtually indistinguishable from normal Bane Spider Executioners. I find using a reference picture helps when going for authenticity.
Technically speaking the banes and crabs use different shoulders (Banes use the less spiky ones, except on executioners), gloves (crabs get the blades, bane don't), legs (banes get the less bulky ones), back pack (crabs get the mechanical arms, higher rank banes get the other one) and helmets (NPC crabs use the stub nosed version which, last I checked is only available as the standard color patern).
I returned to the game recently and I just got a Villain to 50 a few days back.
I rolled a Wolf Spider and at 24 respec'd him to Bane. The issue is that in the tailor I have access to the "Wolf & helm" set and the "Bane/Crab" set but I can only see and select Crab pieces.
So far I've tried:
Going and using the bane powers to see if that would "set" it somehow.
Exiting and reloading the game.
Signing on the Villain that hit 50 again and logging back to the Bane to try see if that re-set it.
I actually just tried making a new Wolf a minute ago and HE had full access to all the Crab pieces but not the Bane.
So, my question is this: Does anyone know why this is happening? I searched the forums, Google, and the known issue list and couldn't find anything, I'm asking here before bugging/petitioning it in case I'm just being really obtuse and missing something right in front of me.
Thanks in advance!