All of the following is my personal understanding of what I see and how I think it was done. I would be grateful for any corrections.
The Czech version that Ray has pointed to may well be similar to the Murano ones, but I think the "ball of spit" as Leni calls it, is not exactly the same. The Czech one seems to be a white core with all-over surface air bubbles - or is it just the photo that gives that impression? A base view might help to confirm this. The Murano ones, as seen in the base view of Leni's weight, have a mass of separate, and different sized air bubbles, in various layers and set over a core of clear glass. [Perhaps this is a form of "Pulegoso" in clear?]
In the Czech weight, the flowers are made from simple sections of blue glass whereas the Murano version uses full millefioiri canes. But millefiori canes may also be used in the Czech weights, too ... they certainly were in Bohemian / German examples of this type of flower weight from the 19th and early 20th century.
The mass of bubbles in the Murano ones are probably formed from addition of such as small pieces of charcoal or chalk in progressive layers. Assuming I am right about the Czech one having a white core with a single coating of bubbles, then the making could be by simlar means but only needing one application of charcoal / chalk.
The general way to produce the flowers is "simply" to pick up the canes (or single coloured glass) and then insert a sharp rod into the centre to stretch the cane / colour downwards to form a stem. In the US versions, this is known as an "Ice-Pick Flower" design. The central air bubble is the natural result of the indentation left after removal of the rod which, after coating in the next layer of glass, causes an air bubble to fill the space. But I am realiably informed that this is a technique that requires skill and speed to ensure the the air buble is not too large, misshapen or even detached from the flower.
Unfortunately, I can't say which of the Murano makers produced weights of this type. But here's a photo (not the best of images) of my "Single-Flower" example, which is of the same type as Leni's: http://tinypic.com/bi04jk.jpg