Just ran across these photos of orchids mounted on a Ginseng bonsai tree
. The orchids attached to the bonsai are Angraecum didieri, Aerangis spiculata and Podangis dactyloceras.
The appeal is really broadened so it's kind of surprising that you don't see this more often. One argument that I occasionally hear against putting orchids on trees is that you can't take them to shows. Well, if you attached orchids to a bonsai tree then you could take the arrangement to both orchid shows and bonsai shows. If the bonsai qualified as a succulent plant then you could take it to cactus and succulent shows as well.
In addition to bonsai trees, there are many common house plants that would make suitable hosts for orchids. If you scroll down this document
you'll find 24 such examples.