I just did my first high gravity sour beer a few months ago, a mega oud bruin to around 1.120. I pitched all the funk into primary, because as you suggested the lacto/Pedio wont go beyond about 8% ABV (many strains of Brett are fine well into the teens). It didnt have much sourness going into secondary, but I wasnt looking to make a really sour ale, I just wanted some tartness to balance the sweet malt backbone.
There is a commercial beer called Mead the Gueuze where they get around the problem by blending the already sour lambic with an already fermented mead. That is certainly an option, as is letting the 6-7% beer sour for a year or so and then adding more honey and fresh yeast to get the rest of the alcohol.
I am planning on starting a regular gravity honey sour ale this fall, probably with the addition of some flowers jasmine/lavender etc
I am still thinking of holding off the honey so that I can feed the beer after the bacteria/Brett has a chance to get moving.