Thanks for your pointers, uboot!
I may have been a bit premature in my judgment of the distributed sources' completeness, although the verdict is not entirely clear yet.
The CONFIG_READER_WIFI symbol is indeed and definitely missing from Sony's tarballs. However, on closer inspection it seems that all other config options shown in /proc/config.gz are actually present in the source, and conversely (and in contrast to what I said in my original posting), /proc/config.gz is also complete with respect to the config options provided in the source. Perhaps the CONFIG_READER_WIFI symbol is not used in any actual kernel code, but only to select other kernel-configuration settings (in a defconfig file that Sony does not distribute)?
Using the source tree, I was able to build a kernel with a matching modversion. I haven't tried booting this kernel, but with it I succeeded in building a kernel module (tun.ko) that insmods and works fine with the firmware's original kernel.