I brewed a huge all-malt barleywine a couple of years ago that is really starting to age well. It started at 1.142 and finished at 1.024 with no additional sugar needed. That calculates to anywhere from 15.5 to 18.3% abv, depending on which calculator you end up using (I may end up sending it out for analysis just for fun). Based on my experience, here's some things to keep in mind:
-You need a highly fermentable wort to make sure it attenuates well. I did an iterated mash for this. I mashed half the grain at 162F for 15 minutes, then pulled the spent grain and added the other half into the liquor and finished at 145F for 90 minutes. If you don't have a way to do this, then I'd mash long and low at 145F or lower.
-Pitch a big, healthy amount of yeast that has been acclimated to higher ABV. Either step up a starter using higher gravities, or just pitch from the cake of a bigger beer (1.070 range). I ended up racking right on the full cake of a 1.068 beer, that had been pitched from some of the cake of a mid 1.040's beer in the previous batch.
-Plan on low efficiency on your system. I usually hit about 82%, but my big boy was in the mid-60's efficiency.
-Oxygenate well, then oxygenate a second time about 12-18 hours later.
-Control your temps well. Keep it low early on, then ramp and rouse at the tail end to squeeze every drop of attenuation out of it.
You seem to be targeting something bigger than what I brewed, so I think the late additions of sugar are a good call. I'd do it in small doses (maybe split up every 12-24 hours over 4-5 doses). I don't know if a second yeast like WLP099 is necessarily required if you take caution, but if you use it, I'd pitch it a small starter at high krausen. I'd also pitch it before you start adding the extra sugar to let the yeast acclimate at a lower ABV and gravity before you start to push it up.
Also, you might want to listen to the episode of "Can You Brew It" where Sean Paxton tries to clone DFH 120-minute IPA using a similar staggered late sugar addition process. There's some good info on that one. You will also want to check gravities prior to each sugar addition to ensure the gravity is still going down. If the yeast have stopped eating, then you're just adding syrup to your beer at that point.
Good luck, and please keep us posted!