Evangelist's caution statement? Political barriers within the core IT organization won't allow for creation of the system, because it doesn't include the other 20,000 employees in other divisions who may need something similar. This is the heart breaking part. The core IT organization cannot dedicate its resources to reduce the burden for 4,000 employees, and lack of trust between a business division and the core IT organization prevents business unit from funding and creating the system on their own. Even if it would eliminate the waste of about 100,000 worker hours a year.
Solution? Get off the ground by bolting on this solution to existing projects. Find supporters in the core IT organization and build personal relationships with them. Continuously consult and involve them in your decision making process even if the architecture has been set. Be prepared to modify your architecture to fit their personal and political views. Achieve early success with core stakeholders, i.e. implement low hanging fruit functionality, and let your users talk up the success of the system. Conduct pilots to demonstrate results and recruit passionate users to be your biggest in the field advocates. This "from the trenches" approach may backfire if not planned and executed properly, but this case may be a good candidate where it's worth taking calculated risk.