It is often desirable to have supports that terminate inside the glass rather than clasping or otherwise holding the glass. However, metal that would survive the firing and be strong enough to support a substantial piece would be of such a size that it would break the glass due to the differing expansion and contraction of the two materials.
The wire or rod to support the fused or cast piece does not have to be incorporated at the point of kiln forming. There is the risk of the glass breaking due to the large differences in expansion and contraction of the metal and the glass, especially with the harder metals.
Instead you need to plan for these supports and keep the glass open at the support points during the kiln forming. It is relatively easy to wrap short pieces of appropriately sized stainless steel rod with fibre paper, or coat with kin wash and build the glass around these, keeping one of each of the ends free of the glass.
You can of course, use other metals, although most – except brass – are likely to spall quite a bit, so wrapping them with fibre paper is best.
When the kiln work is finished, the short rods are pulled out.
You need to clean the cavity formed for the rods. If the glass is transparent or translucent, I like to have the cavity as clear as possible, so I prefer wrapping the rod with fibre paper what ever metal is being used. This provides a clearer surface when clean. Kiln wash leaves a white deposit that is difficult or impossible to clean away completely. Of course if the glass is opalescent, it does not matter whether there is a white deposit.