For non-Wikidata installations, it would be beneficial to use letters other than Q for the Wikibase items. Custom installations still tend to rely on Wikidata as an external reference source, so having two types of Q numbers - those in Wikidata, and those in the custom Wikibase - tend to be highly confusing, especially for a large community-driven project like OpenStreetMap.
For example, if we have OSM-specific items, they should be easily distinguishable from Wikidata items - e.g. Y123 vs Q123.
I have looked in depth at the code, and it seems Q has been hardcoded in a large number of places. Some of these places, like WikibaseDataModel do not appear to have a "settings" context, yet they have ItemId::newFromNumber() methods that use hardcoded Q.
I will try to work on introducing this, but I may need some guidance on how best to do it. Thanks!