The problem here is actually with Blender, since it's Blenders imaging handling that's used. The Octane team would have to write their own "image handler" for Blender to fix this.
(In 2.8 you can actually set keyframes for Offset, that's slightly more helpful).
You can adjust the start frame, but that's just "one shot".
And I agree, there should be a way to have more manual control over the image sequences (for textures), similar to how you can control curve animations.
There's two ways around this, both are less than optimal and rather "botchy":
- Edit the sequence outside of Blender/Octane to match your desired result
- Use several Image Texture nodes, with different Start etc, and then use Mix Texture to switch between them (as you can keyframe the Mix amount)
(NB: Octane actually don't support using movie files for textures, the plugin saves the right frame as an image and sends that to Octane)
(Also, if your using greyscale images/connecting them to greyscale inputs [like opacity], use the "float image texture" [called "Greyscale image" in the Standalone], it uses 1/4th memory [and might speed it up some!?])