I don't know what SAP2i is. Based on your description, it seems to be able to trigger the shutter of your camera. If it can do that, you can try not to use it to dither. Instead, if you go to PHD2's website, you can find a small APP called dither timer. It allows you to dither at a fixed time interval. You can try to sync (manually) the dither timer with your shutter control APP, in such a way that the shutter APP triggers exposure every X minutes and the dither timer also triggers dithering every X minutes. This is what I do for my camera that doesn't have astro APP support for auto dithering.
Hope this helps.
Yes, as everyone's probably figured out by now, SAP2i is the Star Adventurer Pro 2i.
I really appreciated @ktran 's response. I mostly shoot DSOs with the 40-150mm PRO at 150mm, and sometimes 300mm with the 2x teleconverter. (That's 300mm and 600mm, respectively, for you full-frame people.). So, I almost always need guiding.
It's funny. I can connect the computer (with PHD2) to the SAP with the autoguiding camera as an intermediary. And I can connect the SAP to the imaging camera to control the shutter. But PHD2 cannot control the camera through the autoguiding camera+SAP. If that were possible it really would solve this problem. After all, in terms of the electronics, the SAP's ability to control the shutter is nothing fancy. Why do you think this hasn't been implemented? Does the communication standard between the computer+PHD2 and the autoguiding camera not support that sort of command? I mean, it can tell the mount how to correct RA axis rotation, how much more difficult could it be to fire the shutter?