This is a snippet of the script I wrote to remux a specific pin and then expose it to userspace as others have suggested.

    # making gpio 21 an input muxed as a gpio
        echo 0x0104 > /sys/kernel/debug/omap_mux/etk_ctl
        echo 21 > /sys/class/gpio/export
    # making it an output and assigning it a value
        echo 0x0004 > /sys/kernel/debug/omap_mux/etk_ctl
        echo 21 > /sys/class/gpio/export
        echo 1 > /sys/class/gpio/gpio21/value

Note that you can cat any file in that directory and get some useful output. The pin meaning selection is the least significant nibble or two; these files list potential values in numerical order starting with zero on the left. 4 should correspond with gpio21.

> //sys/kernel/debug/ available.  If not, do you recall what kernel option(s)
> I need to set in order to see the omap muxing subtree?

Looking at my config I believe it was CONFIG_OMAP_MUX=y that did it. There are other related options that you might investigate. :) You will then want to mount debugfs somewhere; that's what populates the debug subdirectory for me.

Good luck!