How to link /usr/local to user specific directory?
Yes, you can do it - but it is relatively involved.

Essentially you could chroot each user into his own directory and replicate the filesystem in there, except for the /usr/local directory, which would then be user-specific.

To access the system directories inside the chroot, you could use bindfs, osxfs or even a NFS mount.

Far easier to just let each user have a non-default homebrew and let them rebuild all their packages as needed. By definition, not sharing one folder implies duplicate storage and work but that’s likely far less hassle to set up.