How to send a warning message from git hosting server?

Hello. I am serving a git hosting service for my company.

Sometimes I want to send a warning message to users who use my
service; e.g. the service will be shutdown tomorrow for a while
temporary.

I know it is possible to a remote message by hooks or HTTP body if an
error occured. But it seems that there is no hooks for git-fetch and
git does not print HTTP body if there is no error.

I want a way to response a remote message when a client send any kind
of request. Is it possible?

-EungJun