One simple way to avoid having your rear-most parts slide off the back of the saddle is to simply raise the seat. It can be an extension issue more than seat-tube angle, but there are so many variables. As the saddle is raised, the seat angle will push the saddle rearward. The angle doesn't change, but the relative position behind the bottom bracket does. One thing about saddles: Many similar shaped saddles can change your seat posture dramatically and this is a very person-specific thing. Most modern bikes have 74° or steeper seat-tube angles, so you are limited as to how far the bottom bracket offset can be adjusted.
I know that some of our Alex Moulton models have extraordinarily slack angles—approaching 71° (some are 68°)—but they change when the suspension frame is loaded in compression. I, for one, prefer to use a straight, non-offset seatpost with most of my Moultons, which returns the effective angle/position to around 73°s or so (to match my other bikes), but most people don't bother and are very satisfied.
A custom builder can produce anything for which you are willing to pay, but many will refuse—because of the fear of ridicule for producing an odd design, or the likelihood of perceived future dissatisfaction with an odd design, if extreme.
I hope this is of value to you.
Yours in Cycling,
North Road Bicycle Imports
P.O. Box 840
166 Courthouse Square
Yanceyville, NC 27379
toll free: 800-321-5511