Well, after some random fiddling, I got the /dev/uio0 device to appear, using the parallella-linux kernel. I have no clue why it works now.
However, while I can build your kernel (Xilinx + Epiphany patch), it won't boot. Unfortunately, I don't have a serial cable and can't see why. I only noticed that the Xilinx kernel is much smaller (~2.2 MB) than the parallella-linux kernel (~6 MB), both using parallella_defconfig.
I have looked into the CONFIG_GS_FPGABOOT (or gs_fpga) staging driver, which seems a suitable replacement for Xilinx /dev/xdevcfg driver. However, it doesn't seem functional yet. The xl_init_io() function in drivers/staging/gs_fpgaboot/io.c always returns -1, which results in a "GPIO INIT FAIL!!" error when trying to load (use) the driver. So I have to figure out how to get the Xilinx kernel running. Sigh.
In any case, thank you for your help and the tutorials. I plan on following them as soon as I get the required infrastructure working.