In my opinion, the base malt in this recipe is just a canvas upon which the other ingredients shine. In other words, the most neutral base malt is arguably the best. The crackery, overtly malty character of Maris Otter will, when combined with the relatively large proportions of Vienna and Munich malts, possibly take the beer out of balance. I'd substitute a very neutral base malt, like US 2-row.
You are correct that the ingredient which makes a Belgian-style beer recognizable is the yeast.