I don't think you can. You might be able to recursively check all the files and @ directories on the path for files with the first code line being something like:
classdef XXX < otherSuperClass & mySuperClass & anotherSuperClass
You also would then need to check for
This of course does not mean that XXX or YYY is actually a valid class. There also could be files that look like:
str = 'classdef XXX < otherSuperClass & mySuperClass & anotherSuperClass'
In fact a function that searches for theses classes well likely have such a line. This will make it hard to do the parsing.
There may also be dynamic ways of defining the classes which will be even harder to catch.
A better way may be for the user to have to register the class so the GUI knows about it ...