Since you're using 20% dextrose/turbinado with the WLP099, I think you're OK here. Meph IS pretty thick... does Avery use sugar when they brew it?
How about aeration? What's your plan? Seems like taking it to three steps would give you less of a window to aerate than if you started bigger, aerated longer into fermentation, and then dosed out those 2L of sugar solution at the end (DFH does it this way).
Avery uses Turbinado in all of the big Belgians, or at least did circa 2009 when I lived in Boulder. They don't list it on the website, but I remember the brewer talking about how stupidly expensive it was compared to plain table sugar, but they felt it was worth the price. IIRC at that time they used 3787 for their "Demon" series.
I don't have a way to oxygenate, and I'm hoping that by dividing the fermentables and keeping the gravity low, I can coax the yeast to ferment it out reasonably well. I'm planning on front-loading the harder-to-ferment sugars, so I can use a champagne yeast if necessary to ferment the simple sugar addition.
RJ - Thanks for the feedback and recipe. Definitely food for thought. I'm thinking of using a reverse-mash technique (160*>140*), IIRC like Budweiser uses to brew a highly attenuative wort without additional enzymes.
I've noticed (and Denny mentioned in his article in the latest Zymurgy) when using bourbon, or bourbon soaked oak, that the vanilla in the bourbon exaggerates the perception of sweetness.