Would it work to have an option for the user to choose which shell to use, tclsh or wish? Requiring the 'package require Tk' with wish would be acceptable for that option (at least for me).
Perhaps there could be an optional parameter for both @tcl and @tk, to return the exit code instead of expression result.
Also, @tcl doesn't appear to return the results of an expression. Here's an example:
C:\Tcl\demos\Tk8.5>echo %@tcl[expr 123 + 456]
expr 123 + 456
% expr 123 + 456