Secondment as a Legal KM Tool
I've currently got my nose stuck in Ross Dawson's book - Developing Knowledge-Based Client Relationships. This would be right up his alley. Not only does it develop better client relationships (& could have a Marketing purpose, as Friedmann states), but it could also help firms move up the Commodity-to-Partner value chain.
Another interesting possibility, and obviously dependent on the closeness of the Client/Firm relationship, would be to create a KM Team which included members of both organizations. And if Client Teams were already in place, another angle would be to include a KM practitioner on those teams. Lots of great ways to partner with Clients and provide opportunities to "co-create value". [see Ch. 10 of Ross's book]
BTW, I also did a quick search, and found out that Ross has a pretty good blog.