get back root with custom rom
The SuperSU bundled with CyanogenMod 7 was somehow inaccessible to user root access requests, like with SSHDroid, Terminal Emulator and Midnight Commander, as I've mentioned.

Forums confirm superuser permissions being denied to everything, and inability to get to its configuration. This was a later CyanogenMod design decision.

What finally worked for me was copying the latest CWM-SuperSU-v1.93.zip over to my SD card, rebooting into the "recover" mode CyanogenMod provides, and flashing from that ZIP.
http://www.adbtoolkit.com/rooting/su/zip/#.UxmWtNxdVeQ

Now, I can finally do things like clean out large files from /data/dalvik-cache/*.dex when internal space is badly needed temporarily, or other UNIXy things.