Rear curtain sync only applies to shutters with a rear curtain (i.e., focal plane shutters), and last I checked, that wasn't a feature available on the Speed Graphic.

As for "slow sync," sure leaf shutters sync at any speed with electronic flash (barring a few instances with studio lights at full power, where the flash duration may exceed the highest speed of the shutter), but the flash will still happen at the beginning of the exposure.

It might be an interesting project to attempt to modify a leaf shutter so that it syncs at the end of the exposure.