Of course they are only able to do this because we are quite a small library that's not really open to the public. This, I think, makes it much easier to create opportunities for staff to try working in areas that are not truly part of their job specifications.
I do have experience of volunteering from the other side of the fence, we have taken on a number of volunteers during the time I have been there. They primarily tend to be library students or recent graduates looking to gain that all important experience, but we have also had staff from other departments and Museum Friends. Reading through Headstrong Way's report on a presentation about Bishopsgate Library's use of volunteers I was struck by how similar their set up sounds to ours. Our volunteers tend either to work on particular projects, or if they are library students, get a taster for the different aspects of working in a museum library. The Museum has a volunteers policy in place to make sure everyone (Museum and volunteer) gets as much benefit from the experience as possible.