Have made a quick test and have created a shader over the plugin manager which also generates a spdl file.
This has also no viewport preview. But if i add some paramaters from the txt2s-image-explicit.spdl into this shader, i have a preview.
Now i must find, which of the parameters are needed for the preview and then i can add they into to Octane image shader definition.
I think this would work.
It´s not a guarantee, but it looks not so bad
face