This would work in a theoretical sense, You can add the switch and put the pickups in series ahead of the pre. However, as noted above, you'l also have to turn the blend all the way out to favor the whichever input get's the signal - not real practical for stage use.
If you really want this to work - you need to lose the blend and order the pre with Vol, Vol.
Having lived with a series / parallele switch for a bit - in my opinion - a waste of effort. A large pre-defined volume boost isn't generally something I find usable in live playing - volume boost yes - but it's not always the 3 - 6 - 10 or whateve db jump you get off the switch ... Song to song, ensemble to ensemble - it varies Better to hold some gain in reserve and use that and your own dynamics to control the boost.
As always YMMV. Go fot it! Make it Tort. (gotta cover the basses you know ...)
NOTE: OK - I'm rethinking this - I still don't like the Series parallel pickup approach - it's totally overhyped as a feature in my book but - I'm not so sure the blend control is an issue. The question to be answered is ... Is each pickup at 50% with the blend in the middle or is each at 100% ? Either way is 'balanced/blended' ... my suspicion is that it is 100% which case the blend is irrelevant. You still get the dramatic, uncontrollable not very useful - volume boost ... with no need to touch the blend knob. Essentially you made a kill switch for the other side...
(yeah, I know.. don't hold back .. tell us what ya really think

)