can an http request be altered on the client side ( with a browser, preferably ) to modify the value of an httponly cookie?
Yes, and it's trivial. A cookie is in complete control of the user. He could just install an extension to view/edit cookies, or to tamper the request.
Remember that the cookie is an user provided information, and don't blindly trust any cookie. If you need to store user's data, store it serverside, inside a session, and forward only the session cookie to the client. With enough entropy, bruteforcing the session cookie will be close to impossible.