"info vars" returns the variables from the current context. With the right upvar, or with a specified namespace prefix one can get information about variables in different contexts.
The questions is, whether you want to protect against incorrect usage or against intruders. If your system has already an intruder, that can execute Tcl code, you are in a bad shape, since every proc/cmd you define for protection can be reprogrammed on the fly to behave differently, functions can be renamed, variables can be altered, etc. Also all of the Tcl builtins can be altered. For untrusted code, the most promising approach for Tcl is to work with safe slave interpreters [1], which provide a limited environment, containing just a subset of the commands. However, don't expect that much of OpenACS will run there.
So, it is primarily important to protect against intruders. Over there last weeks i have done some work with open source and commercial vulnerability scanners on openacs (some people might have observed some reboots and worse response times as usual of openacs.org). As a result, i have already submitted a series of security updates, and more will come.
all the best
-g
[1] http://www.tcl.tk/man/tcl8.5/TclCmd/interp.htm
