I had also tested with pathlib and posixpath and come to the same conclusion.

As suggested by you, I looked into `activate` determining path when run. I believe this should do the trick (My bashfoo isn't strong, this is mostly from https://stackoverflow.com/a/179231):

pushd . > /dev/null
SCRIPT_PATH="${BASH_SOURCE[0]}"
if ([ -h "${SCRIPT_PATH}" ]); then
  while([ -h "${SCRIPT_PATH}" ]); do cd `dirname "$SCRIPT_PATH"`; 
  SCRIPT_PATH=`readlink "${SCRIPT_PATH}"`; done
fi
cd `dirname ${SCRIPT_PATH}` > /dev/null
cd .. > /dev/null
SCRIPT_PATH=`pwd`;
popd  > /dev/null

VIRTUAL_ENV="$SCRIPT_PATH"