I guess it doesn't work since you forgot about dependencies. If the desired package needs ithers, it will try to tazpkg get-install them. If you don't have a working connection, that won't work and the package might be installed but not working.
I chose a similar way, installing small to medium packages just in time when i need them instead of at boot time. I added some specific sudo-rights for my user and added a script in /usr/bin which is called by symbolic links that identify the desired package. I put all packages incl. dependencies in a directory, relink /var/cache/tazpkg there and tazpkg GET-install the package. Since it already resides in the (linked) cache, it's installed without being downloaded again.
For bigger packages (Acroread, Skype, OpenOffice, Thunderbird, Seamonkey, Java), I put together the package with some of the needed libraries and put them together in a squashfs-file, which I simply mount and start the package from. By that, no timeconsuming installation is required but the package works out of the box....