I also suspect you will need a DPDT On-On-On to have this set up so that the blender only works when the switch is in the center position. But I'll let others take a run at this, if anyone has any creative way to make this happen . . .
Actually, you don't really need a pickup selector at all if you have the blend pot- but I understand the desire to retain the switch for a quick change in live playing.
I don't know that we have an existing diagram hereabouts, but again, I'll let others weigh in. That is, assuming you're not wedded to the stock switch.
EDIT: And, before you run out and buy the blender:
Wolfs diagram, if adapted to an LP type guitar, would have the two halves of the blend pot wired to the two pickups, then the 3 way switch, then master tone and volume.
The blend pot that is needed is a dual gang pot, with the two tracks specially designed to go 100%, 100%, 0% on one half, and 0%, 100%, 100% respectively, so at the mid position you get 100% of both pups. It means you alswys get a direct connection to at least one pup, while the other gets faded down.
There are issues though. It is effectively adding another pot to load each pickup, hence diminshing treble. Also, as you move away from the centre, I would think the reduced pup will fade away rather quickly.
However, it seems to work on basses as you say.
If I was doing this, I would use an active stage like this: Active Blender
and an on-on-on switch before it to connect the pickups.