How to "flash" a single updated file with CWM Recovery?
After a number of attempts, found this distro at GitHub:
update.zip

Had to replace updater-binary to another version matching my phone (took it from verified and working firmware).

Next, modified updater-script to look like this:

ui_print("________________________________");
run_program("/sbin/busybox", "mount", "/system");
run_program("/sbin/busybox", "mount", "/cache");
run_program("/sbin/busybox", "mount", "/data");
delete("/system/lib/libXXXX.so");
package_extract_file("system/lib/libXXXX.so", "/system/lib/libXXXX.so");
set_perm(1000, 1000, 0644, "/system/lib/libXXXX.so");