I agree with your idea, only output stuff if notify=1 in the config.ini for hijack.
[this lets us end-users configure our config.ini ourselves and set either notify=1 in the normal player section of config.ini and have the player S/W output the notify info as it does now, or notify=0 in the normal place and notify=1 in your section to have the modified version output which includes the fid].

You of course always build the /proc entry for current fid, playlist etc etc -regardless of players notify= entry - that way software using your kernel will always be able to rely on the information there.

For apps that use the serial port themselves, they simply ensure/check that notify=0 in both places in the config.ini where it could appear.
[and presumably fail softly/warn the user if they are not both 0 as it would be fighting with another data stream on the serial port if it ignored these settings when either or both was 1]

My 2 cents.