I understand it is a popular feature request, but the inclusion of multi-user apps would require a refactor of our installer mechanisms in general, as currently there is no way for sub-users to install packages. All packages assume master user and we would need a way to differentiate between multi-user apps and admin apps if we were to expose the QBPMC to your subusers.
The reverse proxy is certainly one solution but it will require a separate endpoint address for each user (quickbox.io/sickrage.liara quickbox.io/sickrage.jmsolo, etc)
While simple on the surface, and yes we could assign random ports during install of each of app the inclusion of this feature in general is a logistical nightmare that will take both @JMSolo and myself refactoring the web panel and installer packages together. While not a full rewrite, I can see the possibility of sinking in many hours of work just to get this into testing.
There are further considerations to the multi-user aspect; such as, the desire for these features will largely apply only to web-hosts looking to add features to shared environments and increase the value of their service. While it would be great, bare in mind we are a two person team who donate the majority of our time. Further, catering to resellers is not a high priority especially given the fact that we do not license our product -- that said, if such a feature does get implemented there is a possibility that it will not be open-sourced and freely available as a part of the main repository.
That said, reverse proxying single-user apps is not a horrible idea and could certainly be done in its current state.