All the riser does is take all of the PCIe pins including power, pass them through a flat cable and terminates them into another connector you plug the GPU into. As long as the GPU can tolerate the additional I*R voltage loss across the cable, it should be fine. Only way to find out for sure is to try it.
If the riser is well-made, the PCIe slot can be extended by a somewhat ridiculous amount. LTT tried it for fun a year or so ago and they put the GPU something like 10' away from the PC before it quit working.