I am wondering if I can deploy a shared installation of Houdini Apprentice and through some environment variable $USER in $HFS/houdini/Licensing.opt direct each user to pick up their own licenses ?

e.g.
localValidatorDir = <path>/$USER/sesi/licenses

You could use environment variables to specify localValidatorDir. The problem is that each Apprentice license is tied to a machine itself, not to the user. So the license file would need to contain licenses for every machine that the user would ever log on, which might probably not practical.