So I broke out the metric tape measure, plumb bob and the handy dandy calculator and did a little geometry.
My bike as I measured it has a 73.29D seat tube angle, however as it sits it has a small setback in the seatpost, so measuring at full extension on a 300mm seatpost from the bottom bracket to the clamp location at the rails I have an effective angle of 72.17 degrees. The post has about a 15mm setback which gives the clamp a location 245mm back and 800mm from the rails to BB. Just to answer the quick question...yeah I tried reversing it and I was sad to say it wouldn't work, couldn't get the saddle angle anywhere near ride-able.
I slid the seatpost out past the min insersion line just to get a feel for what total distance I need, taking into account saddle height and all of that I need another 25-30mm distance away from the BB. Using that number I can then mark where my seat post must insert to (below the weld at the bottom of the top tube and seat stays on the seat tube. From there I set up a line showing 80 degrees the maximum angle one would use for a Tri and measure what my seat post length would need to be at the rails to get the height I need and come up with right about 350mm. So I know I can't go any shorter than 350.
So, knowing what distance I need from BB to rails, and what angles I want I can then set up a triangle and using the angle and hypotenuse solve for the base, which is the distance back the clamp would need to be from the bottom bracket. I can solve this for all the angles I may want, which I did starting with the measured seat tube angle and then additional angles and came up with...
73.29D = 239
74D = 229
75D = 215
76D = 201
77D = 187
78D = 173
79D = 158
80D = 144
So...if I subtract those numbers from 239 I would get for a 0mm off set, I get the needed offset for any given angle I want.
74D = 10mm
75D = 24mm
76D = 38mm
77D = 52mm
78D = 66mm
79D = 81mm
80D = 95mm
Now the last step is to account for saddle adjustment, this is a little more arbitrary as my sit location is not necessarily located at the center of the rails, but I can slide the saddle forward. I currently have it all the way forward which helps some but it ends up putting my sit location only slightly forward of the clamp. But...that being said the rails are about 80mm long, so if your reference point were to be the center of the rails I could in essence pull 40mm from those rails, although I think a number like 20mm is much more realistic. Since this is all relatively fuzzy I have a hard time taking it into consideration when sizing a post because any given saddle will always have adjustment on it and I am concerned with changing the implied frame geometry. Based on some estimations and basic measurements the saddle adjustment seems as though it could potentially get me as much as 2 degrees more upright.
So, after all of this I can now say with some mathematical certainty that anything shorter than 350mm would not do, and I have to essentially maximize offset with my seatpost choice....or as suggested go custom.
So my current short list I have found all seem to come up with less than I would like, but there are some options. The longest only seems to give me 25mm, which would only get me to about 75D, but with a little saddle adjustment I may be able to get up around 76-77 and just have to be happy with that.
Thomson Elite Setback
31.6 x 367 w/16mm offset
Bontrager Race X lite ACC
31.6 x 400 w/20mm offset
FSA SL-K
31.6 x 350 w/25mm offset