Wow, that is really a bad situation and sounds like you are going to be very limited. When I mentioned limitation, I was referring to things such as Google Analytics Admin. Some of the platform sites have their Google Analytics account Admin, but will not give Admin and GWMT access to 3rd parties (marketing agencies). They package in Analytics with their platform monthly rates and set Analytics to mail a report to the client each month. I have had developers refuse to add a video sitemap. I have had some that gave me access to their site backend, but it was hosted on an IIS server and required them to do the manual 301 redirects, and they refused to do this. These are just a few examples of "limitations" I have seen.
Having said that, I really feel your pain here. In your initial question, you called this a "great prospective client," but it is not a great situation as there seems to be a bad breakup with residual bad internal politics. Without knowing your entire situation, here are my recommendations based on what you have mentioned so far:
Option 1 is to not accept the project as it might be too draining and time consuming because of the company politics.
Option 2 is to start by placing proper expectations with the person hiring you - sounds like you have established a good relationship with them. You have to let them know how important on-site optimization is and how the IT dept seem to be unreasonable. Without onsite optimization, your entire campaign is going to be inefficient - it will take a ton more work offsite in order to get results and it is not ideal. Even blogging sounds like a serious pain in that it has to go to a test server first, then you have to wait for an IT person who doesn't "get it" to approve. You should also consider raising your rates because of the situation. This is not typical, so your typical rates that you may have quoted may no longer apply now that you know the whole story.
Anyway, good luck with this. I hope it works. I would love to hear how it plays out.