This is not "minor".

Actually it isn't, that probably was just overlooked.

Any plans to include this feature in next releases?

I wouldn't count on it in 6.x at the very least. While there are numerous changes in Hibernate 6 that are building the foundation where we can consider Embeddable inheritance support, it's something that is out of scope at least for the initial release. Its also worth mentioning that HBM XML is considered deprecated, so seeing actual support for it within HBM will be highly unlikely anyway. The way forward will be either annotations or JPA orm.xml.