I'm using a stock Thomson Elite seatpost until I get a seatpost with setback, per my professional fitter. In the meantime he setup my saddle like this. You can actually see in the photo that the rails go past the "STOP" line. I asked if this was safe to do, and he says yes.
I weigh 160#. I don't want to damage my custom frame if by having the saddle this far backwards will somehow put too much pressure on my seattube, possibly cracking it or something.
It's fine. Whether the seat post is straight or set back makes no difference for the frame, and it is more than strong enough to take that much set back, and much more. I'm actually hard-pressed to think of a situation where saddle set back would be likely to damage a frame before anything else fails.
You might worry about the saddle rails, but I wouldn't; you can see that the rails are well supported by the wide cradle on that seat post even if it does go outside of the recommended range. At your weight, you could slam it even further back on a seat post with a narrower cradle, and it would still be fine.
I have a Thomson seat post, too. I notice that the markings don't mean much when used with that seat post. That's actually little range there. You also managed to get "fecal" in the picture.
Anyway, think of it in terms of lever-arm or how much pressure a longer set of rails would add. It wouldn't be much. I think your seat tube is safe and at your weight, your chances of cracking the rail at the bend is pretty remote.