Actually, much of the code is included in Devel::Init. After the __END__ line is even the code I decided to leave out of Devel::Init that tries to provide compatibility with the first entrant, Attribute::Handlers.

No, Devel::Init is not a tool for enabling developers to easily define their own attributes. But the code in Devel::Init only requires small additions and minor reworking for it to become that.