I've carbed year old 3.4 pH lambics with just sugar in three weeks. You can't be sure the sacc is dead. Also if you bottle that soon with only Roeselare you could end up with bottle bombs. Jolly Pumpkin dregs work fairly quickly though.
You could probably use a yeast that is a high glycerol producer. I don't know the levels that beer yeast produce though. This page has a list for wine yeast though. You could primary with one of the strains that produce a lot of glycerol then add roeselare or some dregs.
In my higher alcohol sours I like to give lacto about four days without any other bacteria or yeast to compete. This way I can get sourness before the alcohol gets high enough to impede the lacto. This would probably work in this situation as well.
I have three five gallon batches of imperial flander's brown that come out to 11,12 and 13% I started the 11% one with lacto only for four days at 90* then lowered the temp and added roeselare. The 12 and 13% I just used second gen roeselare that had Cantillon, JP and Hanssens dregs added during...
I made a sour brown with second runnings from a black IPA and added some home made candy syrup to boost the gravity. It turned out awesome you could really taste the syrup. I will prime with any syrup that has a decent amount of sugar in it. I've been working on a spreadsheet lately that will...
I always like to add some yeast when I pitch started dregs. Just to make sure the bulk of the fermentation will get done quickly enough to fight off anything that will taste bad. Belgian strains are supposed to be good for producing esters that will be transformed into the funky stuff you are...