I’d argue that a series of brown bags or exit interviews don’t do enough–at that point the knowledge transfer is scrubbed, edited, and polished, or lacks the context of when to use such critical experience that others simply don’t have.
The ability to see, archive, and use what others have worked on is most helpful in the situation in which the problem arises. If you’re already introducing Flipcams and YouTube, you may very well have a culture where people are already used to new social sharing tools.
Get people to share their work and their files in secure places but in an open environment; if a boomer speaks at a conference, make sure her slides make it onto Slideshare or another similar service, and use the comments/discussions functionality liberally. Perhaps even some non-sensitive work can be shared on other consumer platforms to introduce a sense of comfort and familiarity into the sharing process that can be called upon for more sensitive or secure documentation, processes, or projects later.
If a retiring boomer has connections with colleagues who you don’t know, ask them to comment or discuss her work as well to begin engaging with that untapped network before it’s too late.
You’re then capturing that focused knowledge in action, and not asking them to recall what they did months or years after the fact.