As of RT-24993 Igor says " Later on we can consider extending set of "special" options to pass custom JVM arguments (e.g. increase heap size by altering desktop shortcut command). But this is separate effort. ".

I agree there's a need for such a support. One the app is deployed on end user's machine if he wants to increase say the heap he may want to add a -Xmx<value>m to the properties of the desktop shortcut. This seems to be the more natural way to do it, especially on Windows.
Another approach is for the user to set a given well known environment variable the launcher would recognize: not sure this is as interesting.