I did a few things to my Master System...
Disabled the onboard RF unit and used the CH. 3/4 switch to control 50/60Hz modes, CH. 3 is NTSC and CH. 4 is PAL. I also used the RF output jack (RCA) for mono audio output.
I also wired up an RGB dongle for the unit, I'm not big on the idea of hacking holes for switches or extra ports.
A few pictures...
(http://img389.imageshack.us/img389/5741/5060lead7hc.th.jpg) (http://img389.imageshack.us/my.php?image=5060lead7hc.jpg) (http://img414.imageshack.us/img414/1103/output3vu.th.jpg) (http://img414.imageshack.us/my.php?image=output3vu.jpg) (http://img414.imageshack.us/img414/1746/smsrgbdongle5ry.th.jpg) (http://img414.imageshack.us/my.php?image=smsrgbdongle5ry.jpg)
I had done the same thing a few months ago with the RF unit on my NES as well, the channel selection switch controls the overclock circuit (CH. 3 is normal and CH. 4 is overclocked). The RF output jack (RCA) is now the "left" audio channel from the CPU (Stereo output mod).
This is the direction I am going with all my consoles that have RF output.
Thanks.
That's a pretty good idea, actually. No one's ever going to use the RF again, might as well use it for something.