-Q9I- grab the command line whenever a non-built-in call is made?

From: -IX-  How, in Tcl, can I XXX:

A9I. The procedure "unknown" is called automatically with arguments 
containing the command and its arguments for any command that couldn't be 
found.  In fact, Tcl and Extended Tcl use this feature to provide demand
loaded commands, and even entire libraries.

So by modifying the unknown procedure you can provide your own extended
functionality, or even remove the demand loading capability if you so
desire.