"supporting organisations in London that hold collections of moving image material. It helps researchers and the public find these collections and tries to ensure the preservation of important material made in or about our city"
That page notes that "Film and video is barely mentioned in our current collecting policy", which is also true for multimedia more widely. Stumbled over it whilst googling for any digital collecting policies from museums. Few turned up, though of course libraries do, and there are odd statements from museums about digital material, but nothing very concrete.
This despite the fact my own university offers a module on the MA course looking at "particular issues of digital collections and digital collection management". I guess the problem is partly that I'm distinguishing between a lower-case "collection" of digital assets, and acquisition and accessioning of digital material into the upper-case Collection of the museum. Where the MLA have looked at "digital collections" they mean digitised collections, quite a different thing. Well that's not quite accurate (see here) but it's a somewhat different perspective, and certainly not guidance on a digital collecting policy as such. Netful of Jewels was the same: talking about collections of digital assets in the lower-case "c" sense.
A couple of exceptions: Denver Museum of Natural History's policy
Wellcome Trust's Library's preservation policy
And of course these folks at least have a digital collection, even if I haven't found the policy: http://www.brooklynmuseum.org/research/digital-collections/brooklynbridge/
Still reading Karen Verschooren's dissertation which includes others, so I know there are some out there. Google just hasn't got them all in its brain.