Hmm, it would seem the default target attribute specified on the Project node is not a valid value in the "BeforeTargets" attribute of Target element.
Specifically, the project file has this top-level Project element defined:
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
I used this "Build" value in the BeforeTargets (see original post), and it is ignored / not recognized. Instead, I'm working around this by using the following:
<Target Name="Browserify" AfterTargets="PrepareForBuild">