This area is tricky, and it is not unlikely to get a solution that works for your application, but which kills others. It would be certainly useful to have a test set exercising host-node-map interactions.
You might get some input from "get_register_subsite" [1], where one has to distinguish between a host-node-map entry pointing to a subsite vs. a host-node-map entry pointing to an application (maybe useful in your case?). This is not only related to URLs for admin, but as well for register redirects etc. Technically, it is also possible to have nested subsites.
To evade the complications for simple applications, xowiki has a feature to strip a prefix to omit unwanted in-between nodes - but certainly, this is only for xo* applications.
[1] https://openacs.org/api-doc/proc-view?proc=security::get_register_subsite&source_p=1
