I've found an article specifically discussing brewing and it carried out various microfermentations with different boil times to demonstrate the apparent extract difference of fermentation (among many other things). The boil times were 30, 45, 60, 90, and 120. Anyhow to summarize the trends in the paper the longer the boil time the higher the final gravity. Instead of splitting hairs and saying whether caramelization or maillard reaction, longer boil will contribute to a higher FG for your souring bacteria/brett yeasts to ferment. This paper also mentions "HPLC analysis did not show the amount of unfermentable sugar fractions present in wort, therefore, effect of wort boiling on sugar levels cannot be ascertained." However I think we can glean from the data that a longer boil increases terminal gravity, that's the only point I'm trying to make. The degree of fermentation possible by the bacteria/brett would depend on the unfermentable sugars present. I believe that the use of caramel malts, or a long boil will produce a higher amount of non-fermentable sugars in a sour beer. Remember even with an apparent degree of fermentation at 100% or more there are still plenty of sugar and carbohydrates remaining in the beer. That would be evident by simply leaving the bottom of a glass of lambic to dry over night into a sticky syrup.