I don't actually know if it's possible, but looking the commit activity, there's a lot of fixes related to e.g. geoclue positioning, and just using upstream 5.6 might not work well on all devices and trying to port those patches to 5.6 might be difficult. Additionally, there are API changes between 5.2 and 5.6, which means upgrading would break many apps written for 5.2. The API changes are small and it's easy to adapt to them, but it might be difficult for app authors to support both versions.

Also note, that this problem of using using a custom tile source, e.g. OSM Scout Server, is only partially served by the OSM plugin in QtLocation 5.6. It allows you to set the host, e.g. "http://tile.openstreetmap.org/" in "http://tile.openstreetmap.org/{z}/{x}/{y}.png", but the requested URL will always end in "{z}/{x}/{y}.png". What we'd really need is a plugin that supports a template URL string, so that you can have X, Y and Z as URL parameters instead of path components or additional parameters like day/night or API keys etc. And also, a plugin parameter for tile size so that the requested tiles can be shown correctly. A custom QtLocation plugin would be easy to distribute separately, e.g. at OpenRepos, but probably difficult to use in Harbour apps.