Apparently this rule is outdated and should not be applied to C# 4.0 and above. See this discussion for similar question.
I would say just disable this rule, as it makes no sense in MVC4.
However, suppressing the rule might not be compliant with your company rules, then you'd have to work around that. (These rules are silly, when you have to hack around some outdated crap like that!)
One of the options is to have a custom rule for the every case like that: specify default value for optional param. (Sorry, can't find implementation for that quickly)
Or, as already suggested go with nullable boolean
public ActionResult OrderBrochure(int brochureId, bool? isModal)
and then in your method check for null on isModel
.