Thanks for the tip on how to make this change more permanent in my installation.
I agree with the idea of a transparent proxy, in theory. In practice, however, it hasn’t worked out that well for me. There were a few problems when I tried to use a transparent proxy back a year or so ago (with Squid running under pfSense). The only one that I can specifically recall is that iTunes sporadically had trouble with downloads of paid content. I could disable the transparent proxy and iTunes would immediately start working after having failed for hours before.