Is it possible to customize .aex effect files using an editor like Visual Studio or Brackets? For example, if I wanted to add an extra slider or point control, what's the general workflow for this? When I open an .aex file in Brackets the code is scrambled and in Visual Studio it just launches Ae.
The short answer would be no; since plug-ins are a beast made of several files, you can't just bring them into Virtual Studio as is, but if you're interested, NTProductions is releasing a tutorial on plug-ins tomorrow that might help you with this.
Also, depending on what you're trying to accomplish, making a pseudo effect might be an easier approach.