I am about to add this blend control to my BB415, and while wondering how it works I have thought of a way to add a "bonus" output via stereo jack.
I added a picture of how I see it electrically, and describe it below.
When pot is fully rotated into one or another direction, isolated pickup is essentially connected to remaining pin, via small resistance (if cut is done in position that allows outer viper to "land").
I want to connect currently not-connected pins together, load them into 220k resistor to emulate usual volume pot wide open, and connect that to ring of TRS jack.
When mono jack is inserted, pickup is grounded from both ends. When stereo jack is inserted "isolated" pickup is actually on ring.
Since I already use no-load pot (made in same manner) for tone, I expect signal to be pretty consistent in stereo with tone wide open.
Will try to do it, and post results.