I was able to figure out how to use OpenCL on the DE0 through a combination of Intel FPGA SDK for OpenCL - Intel Cyclone V SoC Getting Started Guide and a "BSP" (Board Support Package), for the DE0, found thanks to a key forum post.
The Intel documentation hints that OpenCL programming can consume a lot of FPGA logic by including this cautionary message: "The Cyclone V SoC FPGA... is not considered a large FPGA. However, if you structure your kernel code in a way that optimizes hardware usage, it can provide sufficient hardware resources to implement complex computations. Intel recommends that you consult the Intel FPGA SDK for OpenCL Best Practices Guide to obtain a good understanding of the Intel FPGA SDK for OpenCL Offline Compiler, and for strategies on area optimization."
OpenCL: OpenCL Mandelbrot Demo on Atlas-SoC