According to Function Precedence, .mlapp files are above .m files. This means we can create an .m file with the same name as our App, and put the help content in that .m file.
The caveat is at the command window:
returns the not-so-helpful "MyApp is a class." However,
forces MATLAB to look at the .m file rather than the .mlapp, and displays the help desired. This shouldn't have any run-time side effects, and only puts a small burden on users to explicitly add the ".m" to the help command input.