I would like to render an openGL preview using my rendering network, but there is no such option in the rendering engines dropdown. What can I do?
I built the rendering network with "Network Render" addon which is working fine with other engines.
AFAIK netrender doesn't support this, as there is no opengl "render engine" defined in blender (as you noted). There is an opengl render operator, but it is not defined as a formal render engine.
However there is a simple alternative approach you might be able to use. See the following questions: