Hi Todd, around here if the CCC or prisoner/CDF crews are making split rails they run a groove along the length of the log using a chainsaw. Then they insert wedges along the length and start driving them home. some do it in a staggered sequence but I would think if a guy is tough enough he could just pound down and back and get a smaller log to cleave.
Those of us with splitters should be able to make them by rigging adaptors to the basic splitter. One end must grasp the log with a clamp like device or choker chain, the other must be a wedge that extends higher than the cylinder so the rail can be longer than the stock opening for logs.
My little mtd has a moving wedge, so I maybe could extend the wedge higher, and build a cradle/choker assy that would mount to the end of the beam to hold the log level and secure the end of the log that is away from the wedge end while I start the split. After the first extension of the ram, hold the split open with wooden wedges, hold the log with the choker chain, retract ram, advance the log, secure end away from the wedge with choker chain, and extend the wedge to split farther along the log.
I hope this works for someone, I dont have any way to make it right now, but I reckon it would work.