I finally found another reference to a skeleton "mechanism," patented by Webb: http://www.glassfairs.co.uk/Articles/burmese.htm
near bottom of page. Unfortunately it's just a tiny image with no explanation, and it's hard to say for sure if it's applicable to the vases and pitcher that originally brought this to my attention. As far as I can tell, the type of mold in the patent consists of a bunch of metal plates arranged in a circle with edges toward the center, and it's those edges that shape the object; the rest is open.
"in an open mould the glass would blow out in a bubble until it bursts like a balloon." It seems to me that this would only happen if the glass in the openings remained hot enough to keep expanding. It could be cooled by compressed air to prevent this.
There's also the example of items blown into a metal frame that becomes part of the finished piece.
I should say that I'm partly playing devil's advocate here (bad habit of mine) - I don't have any personal experience glassblowing, so perhaps I should be keeping my trap shut!