I've used an app before that is like firebug for GTK, called Parasite but it can't update the widgets in the sense you are suggesting.
You could try something like:
sudo compiz --replace &
if you're using compiz, that is.
Another thing you could try would be to put the commands to change the theme in a script, and assign it to a keyboard shortcut. That way you could change your .gtkrc, hit the keyboard shortcut and watch as you save a minute of your life.
I did a quick google search and found this thread on ubuntu forums.
The script would look something like this (assuming the theme you want to end up with is Ambiance, and the icon theme is ubuntu-mono-dark):
#Change GTK theme:
gconftool-2 --type=string -s /desktop/gnome/interface/gtk_theme Clearlooks
gconftool-2 --type=string -s /desktop/gnome/interface/gtk_theme Ambiance
#Change Metacity theme:
gconftool-2 --type=string -s /apps/metacity/general/theme Clearlooks
gconftool-2 --type=string -s /apps/metacity/general/theme Ambiance
gconftool-2 --type=string -s /desktop/gnome/interface/icon_theme gnome
gconftool-2 --type=string -s /desktop/gnome/interface/icon_theme ubuntu-mono-dark
You may still need to close and re-open applications for the changes to take affect.