We have made solid progress on a very neat integrated process tool including process models, deliverable templates covering the full lifecycle from the point where one gets the contract to go-live. I am not clear in my mind how to deal with product integration. If I was a large system integrator, I would expect it would be all about taking the big software components and making them work together. In our case, I feel we have no significant product integration, because our method is always integrating the new item with the old as part of our normal lifecycle. Therefore, I would like to say that product integration is not relevant to us, but suspect that is not possible?
Well, let's start at the end first. You're correct, Product Integration cannot be "out of scope" for purposes of a CMMI Appraisal. In other words, if your goal is to achieve CMMI ML3 performance, then PI is a "must have." The only process area that can be out of scope is Supplier Agreement Management (SAM) and ONLY of you truly don't use suppliers.
That said, it's interesting that so many people overlook what they are already doing when it comes to "developing processes." I have not seen your system, but based on your description it sounds as if it manages the sequence, creates the environment, and has established procedures and criteria (or else it would not work!). If it also validates program-program interfaces, and perhaps even creates some documentation, even better. Didn't I just list more than half of the practices within Product Integration?
Sometimes the "stuff" we do everyday is, by definition, the process. You may need to round it out with some process definitions, guidelines for tailoring, training, and policies, but in the end, it sounds like a "PI Process" to me. With all of the bad tool implementations out there it's refreshing to see an example where a tool actually helped.
I recently conducted an appraisal where the client had a similar system to manage the entire lifecycle. Their system also created the executable package, the documentation, and installed it on the client site (they had a dedicated, long-term client). After digging into it for a couple of days we determined that it (with a few documents) did meet the spirit of Product Integration.
These types of innovative solutions are too few and far between in our business. Documents do not make a process - and the mentality that it takes a million documents to succeed at CMMI is just wrong. Congratulations on a great start.