>My Perl/Tk book implied that DialogBox is a 
>composite widget written in perl (it refers to 
>DialogBox.pm). So I went looking on the web... 

A couple of people on the Tcl newsgroup pointed me to TopLevel, which
is precisely what I needed!  It's been in every book I looked at, but
I was obviously blinkered, because I was looking for something that
mentioned "dialog".

I now have some working code.  I don't know yet how to get it to  pop
up the window in a particular position, but at least I can enter my
text now!

Once I get this program into a state where I wouldn't feel too
embarrassed for other people to see the code, I'll work out how to
post it to the RAA.

By the way, if anyone is interested in editing graph theory diagrams
(ie, nodes and edges) and generating LaTeX from it, feel free to drop
me a line and I'll send you a beta.  I'd love some feedback on the
user interface and any suggestions for extensions.

Thanks for the help, people.