Yep, that is what I did (my example above was incorrect). Here the adapted config from the compile script:
# method
KERNEL_ONLY="yes" # build kernel only
SOURCE_COMPILE="yes" # force source compilation
KERNEL_CONFIGURE="yes" # change default configuration
KERNEL_CLEAN="yes" # MAKE clean before compilation
USEALLCORES="yes" # Use all CPU cores
BUILD_DESKTOP="yes" # desktop with hw acceleration for some boards
# user
DEST_LANG="en_US.UTF-8" # sl_SI.UTF-8, en_US.UTF-8
CONSOLE_CHAR="UTF-8" # console charset
TZDATA="Europe/Berlin" # time zone
ROOTPWD="1234" # forced to change @first login
SDSIZE="1500" # SD image size in MB
AFTERINSTALL="" # command before closing image
MAINTAINER="Igor Pecovnik" # deb signature
MAINTAINERMAIL="igor.pecovnik@****l.com" # deb signature
GPG_PASS="" # set GPG password for non-interactive packing
# advanced
KERNELTAG="v3.18.13" # kernel TAG - valid only for mainline
UBOOTTAG="v2015.04" # kernel TAG - valid for all sunxi
FBTFT="yes" # https://github.com/notro/fbtft
EXTERNAL="yes" # compile extra drivers
FORCE="yes" # ignore manual changes to source
If I did not mess around somewhere else, this does not work and the tag is ignored. I get a 4.0 kernel being checked out.