Update Git super repository automatically when a submodule gets updated

in our company we have a huge code base (>100000 files) and so we keep it in several git repositories. So we have a forest of repositories and one super repository containing only submodule references on top of that.

The idea is to have the super repository just as a convenience glue and update it automatically whenever a developer updates any submodule.

I have experimented with the post-receive hook and ended up with the following implementation:
(it involves git plumbing in order to be able to modify the bare repository directly)

#!/bin/bash -e

UPDATED_BRANCHES="^(master|develop)$"
UPDATED_REPOS="^submodules/.+$"

# determine what branch gets modified
read REV_OLD REV_NEW FULL_REF
BRANCH=${FULL_REF##refs/heads/}

if [[ "${BRANCH}" =~ ${UPDATED_BRANCHES} ]] && [[ "${GL_REPO}" =~ ${UPDATED_REPOS} ]];
then
    # determine the name of the branch in the super repository
    SUPERBRANCH=$FULL_REF
    SUBMODULE_NAME=${GL_REPO##submodules/}
    # clean the submodule repo related environment
    unset $(git rev-parse --local-env-vars)
    # move to the super repository
    cd $SUPERREPO_DIR

    echo "Automaticaly updating the '$SUBMODULE_NAME' reference in the super   repository..."
    # modify the index - replace the submodule reference hash
    git ls-tree $SUPERBRANCH | \
        sed "s/\([1-8]*\) commit \([0-9a-f]*\)\t$SUBMODULE_NAME/\1 commit $REV_NEW\t$SUBMODULE_NAME/g" | \
        git update-index --index-info

    # write the tree containing the modified index
    TREE_NEW=$(git write-tree)
    COMMIT_OLD=$(git show-ref --hash $SUPERBRANCH)

    # write the tree to a new commit and use the current commit as its parent
    COMMIT_NEW=$(echo "Auto-update submodule: $SUBMODULE_NAME" | git commit-tree $TREE_NEW -p $COMMIT_OLD)

    # update the branch reference
    git update-ref $SUPERBRANCH $COMMIT_NEW
    # shall we also update the HEAD?
    # git symbolic-ref HEAD $SUPERBRANCH
fi

Now the questions are:

• Is it a good idea at all to use a git hook to modify another repository than the one that triggered the event?
• Is the hook implementation OK?
  (It seems to be working on my machine, but I have no prior experience with git plumbing and so maybe I have omitted something)
• I guess there is a possibility of race conditions in case of two (or more) submodules being updated simultaneously. Is it possible to prevent that somehow (e.g. a lock file)?
  (we are using gitolite as the access layer).
• Would it be better to use a clone of the super repository for the modification and then push (as opposed to modify the bare super repository directly)?

Thanks in advance.