Does anyone know if it is possible to define an MDK "Custom Tool Menu" item from within a device pack?
No, there is no automatic way to get this inserted e.g. when installing the corresponding pack file.
Note that this tools menu is a product specific feature and CMSIS Pack is supported by various IDEs.
You can simplify the configuration for the user by providing a configuration file that the users can import.
Please keep in mind that the path will keep changing each time you release a new version. Therefore it may be better to install the tool that you want to add separately rather than in packs.
~ Joachim