I have seen that done and I thought it was a good trick.
On a related note a lot of wheels with deep rims, if they do have holes, often have holes that are big enough to let the nipples into the rim but loose nipples are reluctant to come out again through the same holes. The valve hole is often bigger and loose nipples will often come out more easily here instead.
I have found that the easiest way of persuading loose nipples to come out of the valve hole (should they not simply fall out of their own accord) is simply to hold the rim vertical (with the valve hole lowermost) and to bounce the rim repeatedly on a suitable floor surface (with some kind of covering eg lino so that the rim won't scratch) or on a piece of wood. Loose nipples inside the rim bounce around like crazy and soon find their way out. You know when you are done because the rattling suddenly stops.
For some reason this usually works much faster than any amount of shaking the rim by hand.
cheers
