You need to work with the DROP event rather than the SCLICK event. I've not managed to get this working properly (it only drags if your start/end are within the dialog).
on *:dialog:yourdialog:drop:*:{
dialog -s $dname $mouse.mx $mouse.my $dialog($dname).cw $dialog($dname).ch
}
This will cause it to center on your final mouse position. To have it put the top left corner on your final mouse position, use $mouse.dx and $mouse.dy instead.
Still, like I said, I can't make it work if you leave the dialog while dragging. It will at least get you started.