Is it a good idea to put equations in a research proposal?
While I am not a physicist (I'm a mathematician, but this doesn't particularly make me any more partial to equations), the point behind the quote on that website is that you want your proposals to be easy to understand, and not make the reviewer wade through a sea of technicalities.  Since you're writing for experts, if equations make the proposal easier to read and understand, please use them.  Everything should be made as simple as possible...