What if a small practice group at a very large firm were handling a significant number of matters for a company? If that group seceded, and continued on but with much lower overhead and assurances from the law department of a steady flow of work, the newly-formed firm and the law department would both benefit. More profit for the former; lower costs and established talent for the latter. The arrangement would not have to be exclusive.
Since many departments do not have enough work of their own to sustain a break-away firm, another possibility arises. I can imagine two or three law departments combining to promise the fledgling firm a steady supply of work, in return for considerably lower rates and even more devoted attention.