Thank you for your generous reply.
Building a plugin, is exactly what I am doing.
I started with mantis from the understanding of its architecture, and now, I have a well organized plugin working, using all its native components (schema, events, apis, pages, files, configuration, menus...), configuring the business view I need for a set of predefined manteinance services (called "projects" in mantis), matching my business requirements. It has been a pleasure to learn how mantis is done, to reach its high flexibility, reliability, and scalability (I've developed some state machines in the past, and the mantis solves most of the issues in an elegant way), but I am still a bit confused, because PHP not seems as a complete OO programming language implementation, and/or, mantis is not completely developed in that way.
Fortunately: mantis is enough flexible to match all my business requirements; PHP has an infinite development network; and the mantis documentation is complete, very well formed, and the product has a good support. It's greatness synthesizing complexity, has the reasonable price of adapting the business view to mantis paradigms (new business entities, or simply its structure...), which requires an integration solution, where plugin architecture also seems as a great option. This is something usual with software integrators, developing vertical (industry specialized) solutions, which now is the role I have, and the view I have applied to mantis, to decide if it is the core system that my customer needs to control some of its business services.
I am quite sure I am not making a mistake with my decission, and I feel conscient, and assuming the dificulty behind the result I am expecting. The new mantisbt UI, improves the user experience, but when you try to mix information from more than one system, and execute transactions to keep mantis working properly (i.e. user manteinance, where users could come from different data sources), some standard integration patterns and good practices are missed. Http and JSON are good standards for me, but I would like to make public the complete set of my plugin functions (the API section), and I would like to handle mantis business classes definitions and data structures to extend what I need (new business classes consuming new business services). I would like to extend the architecture applied to develop the existing JSON API, but I do not know the impact on the context. Simply, I would like to publish a JSON service
I need to know how to create in my plugin, new custom JSON services to manage my own business entities, integrated whith the mantis ensuring access level, and I would like to re-use the current mantis infrastructure. How?
This need is closely related with the mantis ability to manage groups of users, and could be easyly avoided with such capability, and that is why I have to develop something to paste both worlds. By now, I found three options: a) implement user groups, feeling as capable as mantis core develompent team, to modify the standard behaviour of the security for the whole system (I do not think so); b) wait to mantis team to develop such ability (100% business lost), and c), try to paste both worlds with a consistent process (may be, I will find a reasonable solution which could be performed with my notepad++ code editor and my phpMyAdmin as precarious tools).