i wouldn't underpitch that big a beer. the environment will be stressful enough with all that sugar in there. you're going to need a lot of soldiers to conquer that hill.
i would consider adding sugar in waves after primary has starter. that way you'll start off with a 1.080 (for example) beer, but through the addition of sugar on days 3, 5 and 7 you ramp it up to 1.100.
i don't have any experience brewing a belgian that big, but i'm not sure that there is any belgian ale yeast that can get you to 14%. you might want to start off with a belgian yeast and see where it gets you, and have some high-gravity yeast (wlp099 or a wine/champagne yeast) ready to take over once the belgian craps out. if you go with 099, be sure to get it going in a starter first and pitch at high krausen. you want those bugs active before going into a ~10% ABV environment.
there are several excellent threads on this forum that discuss high-gravity brewing - excellent resources.