Sorry, yes, you are right. The material is shared among all the objects (there is only one Octane image texture node for the whole scene), but it is evaluated for each OBJ node at loading time, so as you say if the reload texture option is enabled, the texture is reloaded for each OBJ.
I will try to fix this issue for the next build. Meanwhile I suppose that a workaround can be have two cloned materials, one with the reload texture option enabled and other with it disabled, and use the material with the option enabled only in one object. This should be enough to reload the texture map for all the objects, loading it only once.
Thanks for reporting this issue!
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini