MOF26 — Incomplete simplification & alignment between UML & MOF in 2.4: MOF::Extension::Tag

The simplification and alignment between UML and MOF in the 2.4 series is incomplete.
In particular, the extensions that MOF adds to UML are missing in UML.
The MOF extensions that are missing in UML mean that statements like the one below in UML 2.5, section 6.2 are technically incorrect:

Since version 2.4.1 a MOF 2.x metamodel, including the UML 2.x metamodel, is a valid UML 2.x model. This was a substantial simplification and alignment compared to earlier versions. It is expected that future versions of MOF and UML will continue to be aligned in this manner.

For example, UML has no mechanism to specify the information about MOF::Extension::Tag. Without this information, it is currently not possible to fully rely on the above statement to use UML as a language for representing models of UML itself or parts of it such as the PrimitiveTypes library.

One fairly simple option would be to define a MOF profile with stereotypes corresponding to the contents of the MOF-specific extensions of UML