As I've mentioned already, it is possible to version control the [moodledata]/lang/en_local folder (we've done this for one customer, for similar reasons), but agreed that it isn't quite as convenient as having all the code in one place.
The feedback situation is unfortunate - it would be a lot better if plugins there could be entirely self-contained.
I guess it is up to you as to whether you prefer to use on production a feature designed only for developer usage that is not (officially) tested during Moodle code upgrades and which has no guarantee of working / existing in the future; OR if you feel it is a greater risk to add a couple of extra lines to the end of one (or more) of the core language files. I'd, personally, go for the 2 extra lines in the language file. However, I have to implement so many customer requests that are really impossible without (small) core changes, that, maybe, I've just got too comfortable with doing that (and letting git handle the maintenance).