Dunno, the only color transparency film I've shot with pinhole was Fujichrome T64 with an 85B filter taped over the front. An equivalent exposure only about 20% longer than a test shot with a lens (same scene, same camera) produced a good exposure match. The info available seems to be a little sketchy, and the tungsten films are apparently optimized for longer exposures, which worked in my favor. With multiple failures, I'm inclined to wonder if your calculated f-stop might be off -- it wouldn't take much of an error in that pinhole diameter measurement to throw things off quite a bit. In the instance I described, I bracketed several exposures about a stop either way. I fear testing is about the only reliable way. My approach would be to run a roll with some bracketed test shots and skip any expensive scanning until you see what you get.
The other hazard with color, of course, is that the different layers may respond differently as you get far away from the optimized useful range. I notice that Kodak sheet said something about a small color compensation filter at long exposures.