You might be able to, with considerable effort, get your
shell to do this for you. However, it's going to be a lot
easier to just set the umask in each of your scripts to
whatever you desire. Alternatively, if the umask you
want for that directory is one you can live with as your
default, set it in your shell's dotfiles. As yet another
option that's kind of crude, you might create a special account with a default umask set to your choice, have it run
the scripts, and use sudo, if you run them interactively.
On most flavors of unix (including Linux) though, you're not going to be able
to do exactly what you want. For flavors that can do what
you want, the capability is often present in the ACL
facilities, so do an 'apropos acl' to figure out what
manpages to look at. Note also that on Linux, ACLs are
probably going to be showing up in mainstream distributions
within the next year or so.